Chapter 8 Language Structure

Table of Contents

8.1 Literal Values
8.1.1 String Literals
8.1.2 Number Literals
8.1.3 Date and Time Literals
8.1.4 Hexadecimal Literals
8.1.5 Boolean Literals
8.1.6 NULL Values
8.2 Database, Table, Index, Column, and Alias Names
8.2.1 Identifier Qualifiers
8.2.2 Identifier Case Sensitivity
8.2.3 Function Name Parsing and Resolution
8.3 Reserved Words
8.4 User-Defined Variables
8.5 Expression Syntax
8.6 Comment Syntax

This chapter discusses the rules for writing the following elements of SQL statements when using MySQL:

8.1 Literal Values

This section describes how to write literal values in MySQL. These include strings, numbers, hexadecimal values, boolean values, and NULL. The section also covers the various nuances and gotchas that you may run into when dealing with these basic types in MySQL.

8.1.1 String Literals

A string is a sequence of bytes or characters, enclosed within either single quote (') or double quote (") characters. Examples:

'a string'
"another string"

Quoted strings placed next to each other are concatenated to a single string. The following lines are equivalent:

'a string'
'a' ' ' 'string'

If the ANSI_QUOTES SQL mode is enabled, string literals can be quoted only within single quotation marks because a string quoted within double quotation marks is interpreted as an identifier.

As of MySQL 4.1.1, a binary string is a string of bytes that has no character set or collation. A nonbinary string is a string of characters that has a character set and collation. For both types of strings, comparisons are based on the numeric values of the string unit. For binary strings, the unit is the byte. For nonbinary strings the unit is the character and some character sets support multi-byte characters. Character value ordering is a function of the string collation.

Also as of MySQL 4.1.1, string literals may have an optional character set introducer and COLLATE clause:

[_charset_name]'string' [COLLATE collation_name]

Examples:

SELECT _latin1'string';
SELECT _latin1'string' COLLATE latin1_danish_ci;

You can use N'literal' (or n'literal') to create a string in the national character set. These statements are equivalent:

SELECT N'some text';
SELECT n'some text';
SELECT _utf8'some text';

For more information about these forms of string syntax, see Section 9.1.3.5, “Character String Literal Character Set and Collation”, and Section 9.1.3.6, “National Character Set”.

Within a string, certain sequences have special meaning. Each of these sequences begins with a backslash (\), known as the escape character. MySQL recognizes the following escape sequences.

CharacterEscape Sequence
\0 An ASCII NUL (0x00) character.
\' A single quote (') character.
\" A double quote (") character.
\b A backspace character.
\n A newline (linefeed) character.
\r A carriage return character.
\t A tab character.
\Z ASCII 26 (Control-Z). See note following the table.
\\ A backslash (\) character.
\% A % character. See note following the table.
\_ A _ character. See note following the table.

For all other escape sequences, backslash is ignored. That is, the escaped character is interpreted as if it was not escaped. For example, \x is just x.

These sequences are case sensitive. For example, \b is interpreted as a backspace, but \B is interpreted as B.

The ASCII 26 character can be encoded as \Z to enable you to work around the problem that ASCII 26 stands for END-OF-FILE on Windows. ASCII 26 within a file causes problems if you try to use mysql db_name < file_name.

Escape processing is done according to the character set indicated by the character_set_connection system variable. This is true even for strings that are preceded by an introducer that indicates a different character set, as discussed in Section 9.1.3.5, “Character String Literal Character Set and Collation”.

The \% and \_ sequences are used to search for literal instances of % and _ in pattern-matching contexts where they would otherwise be interpreted as wildcard characters. See the description of the LIKE operator in Section 11.5.1, “String Comparison Functions”. If you use \% or \_ outside of pattern-matching contexts, they evaluate to the strings \% and \_, not to % and _.

There are several ways to include quote characters within a string:

  • A ' inside a string quoted with ' may be written as ''.

  • A " inside a string quoted with " may be written as "".

  • Precede the quote character by an escape character (\).

  • A ' inside a string quoted with " needs no special treatment and need not be doubled or escaped. In the same way, " inside a string quoted with ' needs no special treatment.

The following SELECT statements demonstrate how quoting and escaping work:

mysql> SELECT 'hello', '"hello"', '""hello""', 'hel''lo', '\'hello';
+-------+---------+-----------+--------+--------+
| hello | "hello" | ""hello"" | hel'lo | 'hello |
+-------+---------+-----------+--------+--------+

mysql> SELECT "hello", "'hello'", "''hello''", "hel""lo", "\"hello";
+-------+---------+-----------+--------+--------+
| hello | 'hello' | ''hello'' | hel"lo | "hello |
+-------+---------+-----------+--------+--------+

mysql> SELECT 'This\nIs\nFour\nLines';
+--------------------+
| This
Is
Four
Lines |
+--------------------+

mysql> SELECT 'disappearing\ backslash';
+------------------------+
| disappearing backslash |
+------------------------+

If you want to insert binary data into a string column (such as a BLOB column), the following characters must be represented by escape sequences.

CharacterEscape Sequence
NULNUL byte (0x00). Represent this character by \0 (a backslash followed by an ASCII 0 character).
\Backslash (ASCII 92). Represent this character by \\.
'Single quote (ASCII 39). Represent this character by \'.
"Double quote (ASCII 34). Represent this character by \".

When writing application programs, any string that might contain any of these special characters must be properly escaped before the string is used as a data value in an SQL statement that is sent to the MySQL server. You can do this in two ways:

  • Process the string with a function that escapes the special characters. In a C program, you can use the mysql_real_escape_string() C API function to escape characters. See Section 17.6.6.51, “mysql_real_escape_string()”. The Perl DBI interface provides a quote method to convert special characters to the proper escape sequences. See Section 17.8, “MySQL Perl API”. Other language interfaces may provide a similar capability.

  • As an alternative to explicitly escaping special characters, many MySQL APIs provide a placeholder capability that enables you to insert special markers into a statement string, and then bind data values to them when you issue the statement. In this case, the API takes care of escaping special characters in the values for you.

8.1.2 Number Literals

Integers are represented as a sequence of digits. Floats use . as a decimal separator. Either type of number may be preceded by - or + to indicate a negative or positive value, respectively

Examples of valid integers:

1221
0
-32

Examples of valid floating-point numbers:

294.42
-32032.6809e+10
148.00

An integer may be used in a floating-point context; it is interpreted as the equivalent floating-point number.

8.1.3 Date and Time Literals

Date and time values can be represented in several formats, such as quoted strings or as numbers, depending on the exact type of the value and other factors. For example, in contexts where MySQL expects a date, it interprets any of '2015-07-21', '20150721', and 20150721 as a date.

This section describes the acceptable formats for date and time literals. For more information about the temporal data types, such as the range of permitted values, consult these sections:

Standard SQL and ODBC Date and Time Literals

Standard SQL permits temporal literals to be specified using a type keyword and a string. The space between the keyword and string is optional.

DATE 'str'
TIME 'str'
TIMESTAMP 'str'

MySQL recognizes those constructions and also the corresponding ODBC syntax:

{ d 'str' }
{ t 'str' }
{ ts 'str' }

However, MySQL ignores the type keyword and each of the preceding constructions produces the string value 'str', with a type of VARCHAR.

String and Numeric Literals in Date and Time Context

MySQL recognizes DATE values in these formats:

  • As a string in either 'YYYY-MM-DD' or 'YY-MM-DD' format. A relaxed syntax is permitted: Any punctuation character may be used as the delimiter between date parts. For example, '2012-12-31', '2012/12/31', '2012^12^31', and '2012@12@31' are equivalent.

  • As a string with no delimiters in either 'YYYYMMDD' or 'YYMMDD' format, provided that the string makes sense as a date. For example, '20070523' and '070523' are interpreted as '2007-05-23', but '071332' is illegal (it has nonsensical month and day parts) and becomes '0000-00-00'.

  • As a number in either YYYYMMDD or YYMMDD format, provided that the number makes sense as a date. For example, 19830905 and 830905 are interpreted as '1983-09-05'.

MySQL recognizes DATETIME and TIMESTAMP values in these formats:

  • As a string in either 'YYYY-MM-DD HH:MM:SS' or 'YY-MM-DD HH:MM:SS' format. A relaxed syntax is permitted here, too: Any punctuation character may be used as the delimiter between date parts or time parts. For example, '2012-12-31 11:30:45', '2012^12^31 11+30+45', '2012/12/31 11*30*45', and '2012@12@31 11^30^45' are equivalent.

  • As a string with no delimiters in either 'YYYYMMDDHHMMSS' or 'YYMMDDHHMMSS' format, provided that the string makes sense as a date. For example, '20070523091528' and '070523091528' are interpreted as '2007-05-23 09:15:28', but '071122129015' is illegal (it has a nonsensical minute part) and becomes '0000-00-00 00:00:00'.

  • As a number in either YYYYMMDDHHMMSS or YYMMDDHHMMSS format, provided that the number makes sense as a date. For example, 19830905132800 and 830905132800 are interpreted as '1983-09-05 13:28:00'.

A DATETIME or TIMESTAMP value can include a trailing fractional seconds part in up to microseconds (6 digits) precision. Although this fractional part is recognized, it is discarded from values stored into DATETIME or TIMESTAMP columns. For information about fractional seconds support in MySQL, see Section 10.3.4, “Fractional Seconds in Time Values”.

Dates containing two-digit year values are ambiguous because the century is unknown. MySQL interprets two-digit year values using these rules:

  • Year values in the range 70-99 are converted to 1970-1999.

  • Year values in the range 00-69 are converted to 2000-2069.

See also Section 10.3.6, “Two-Digit Years in Dates”.

For values specified as strings that include date part delimiters, it is unnecessary to specify two digits for month or day values that are less than 10. '2015-6-9' is the same as '2015-06-09'. Similarly, for values specified as strings that include time part delimiters, it is unnecessary to specify two digits for hour, minute, or second values that are less than 10. '2015-10-30 1:2:3' is the same as '2015-10-30 01:02:03'.

Values specified as numbers should be 6, 8, 12, or 14 digits long. If a number is 8 or 14 digits long, it is assumed to be in YYYYMMDD or YYYYMMDDHHMMSS format and that the year is given by the first 4 digits. If the number is 6 or 12 digits long, it is assumed to be in YYMMDD or YYMMDDHHMMSS format and that the year is given by the first 2 digits. Numbers that are not one of these lengths are interpreted as though padded with leading zeros to the closest length.

Values specified as nondelimited strings are interpreted according their length. For a string 8 or 14 characters long, the year is assumed to be given by the first 4 characters. Otherwise, the year is assumed to be given by the first 2 characters. The string is interpreted from left to right to find year, month, day, hour, minute, and second values, for as many parts as are present in the string. This means you should not use strings that have fewer than 6 characters. For example, if you specify '9903', thinking that represents March, 1999, MySQL converts it to the zero date value. This occurs because the year and month values are 99 and 03, but the day part is completely missing. However, you can explicitly specify a value of zero to represent missing month or day parts. For example, to insert the value '1999-03-00', use '990300'.

MySQL recognizes TIME values in these formats:

  • As a string in 'D HH:MM:SS' format. You can also use one of the following relaxed syntaxes: 'HH:MM:SS', 'HH:MM', 'D HH:MM', 'D HH', or 'SS'. Here D represents days and can have a value from 0 to 34.

  • As a string with no delimiters in 'HHMMSS' format, provided that it makes sense as a time. For example, '101112' is understood as '10:11:12', but '109712' is illegal (it has a nonsensical minute part) and becomes '00:00:00'.

  • As a number in HHMMSS format, provided that it makes sense as a time. For example, 101112 is understood as '10:11:12'. The following alternative formats are also understood: SS, MMSS, or HHMMSS.

A trailing fractional seconds part is recognized in the 'D HH:MM:SS.fraction', 'HH:MM:SS.fraction', 'HHMMSS.fraction', and HHMMSS.fraction time formats, where fraction is the fractional part in up to microseconds (6 digits) precision. Although this fractional part is recognized, it is discarded from values stored into TIME columns. For information about fractional seconds support in MySQL, see Section 10.3.4, “Fractional Seconds in Time Values”.

For TIME values specified as strings that include a time part delimiter, it is unnecessary to specify two digits for hours, minutes, or seconds values that are less than 10. '8:3:2' is the same as '08:03:02'.

8.1.4 Hexadecimal Literals

MySQL supports hexadecimal values, written using X'val', x'val', or 0xval format, where val contains hexadecimal digits (0..9, A..F). Lettercase of the digits does not matter. For values written using X'val' or x'val' format, val must contain an even number of digits. For values written using 0xval syntax, values that contain an odd number of digits are treated as having an extra leading 0. For example, 0x0a and 0xaaa are interpreted as 0x0a and 0x0aaa.

In numeric contexts, hexadecimal values act like integers (64-bit precision). In string contexts, they act like binary strings, where each pair of hex digits is converted to a character:

mysql> SELECT X'4D7953514C';
        -> 'MySQL'
mysql> SELECT 0x0a+0;
        -> 10
mysql> SELECT 0x5061756c;
        -> 'Paul'

In MySQL 4.1 (and in MySQL 4.0 when using the --new option), the default type of a hexadecimal value is a string. If you want to ensure that the value is treated as a number, you can use CAST(... AS UNSIGNED):

mysql> SELECT 0x41, CAST(0x41 AS UNSIGNED);
        -> 'A', 65

The X'hexstring' syntax is new in 4.0 and is based on standard SQL. The 0x syntax is based on ODBC. Hexadecimal strings are often used by ODBC to supply values for BLOB columns.

Beginning with MySQL 4.0.1, you can convert a string or a number to a string in hexadecimal format with the HEX() function:

mysql> SELECT HEX('cat');
        -> '636174'
mysql> SELECT 0x636174;
        -> 'cat'

8.1.5 Boolean Literals

Beginning with MySQL 4.1, The constants TRUE and FALSE evaluate to 1 and 0, respectively. The constant names can be written in any lettercase.

mysql> SELECT TRUE, true, FALSE, false;
        -> 1, 1, 0, 0

8.1.6 NULL Values

The NULL value means no data. NULL can be written in any lettercase. A synonym is \N (case sensitive).

For text file import or export operations performed with LOAD DATA INFILE or SELECT ... INTO OUTFILE, NULL is represented by the \N sequence. See Section 12.2.5, “LOAD DATA INFILE Syntax”.

Be aware that the NULL value is different from values such as 0 for numeric types or the empty string for string types. For more information, see Section B.5.5.3, “Problems with NULL Values”.

8.2 Database, Table, Index, Column, and Alias Names

Database, table, index, column, and alias names are identifiers. This section describes the permissible syntax for identifiers in MySQL. Section 8.2.2, “Identifier Case Sensitivity”, describes which types of identifiers are case sensitive and under what conditions.

An identifier may be quoted or unquoted. If an identifier contains special characters or is a reserved word, you must quote it whenever you refer to it. The set of alphanumeric characters from the current character set, _, and $ are not special. Reserved words are listed at Section 8.3, “Reserved Words”. (Exception: A reserved word that follows a period in a qualified name must be an identifier, so it need not be quoted.)

The identifier quote character is the backtick (`):

mysql> SELECT * FROM `select` WHERE `select`.id > 100;

If the ANSI_QUOTES SQL mode is enabled, it is also permissible to quote identifiers within double quotation marks:

mysql> CREATE TABLE "test" (col INT);
ERROR 1064: You have an error in your SQL syntax...
mysql> SET sql_mode='ANSI_QUOTES';
mysql> CREATE TABLE "test" (col INT);
Query OK, 0 rows affected (0.00 sec)

The ANSI_QUOTES mode causes the server to interpret double-quoted strings as identifiers. Consequently, when this mode is enabled, string literals must be enclosed within single quotation marks. They cannot be enclosed within double quotation marks. The server SQL mode is controlled as described in Section 5.1.6, “Server SQL Modes”.

As of MySQL 4.1, identifier quote characters can be included within an identifier if you quote the identifier. If the character to be included within the identifier is the same as that used to quote the identifier itself, double the character. The following statement creates a table named a`b that contains a column named c"d:

mysql> CREATE TABLE `a``b` (`c"d` INT);

In the select list of a query, a quoted column alias can be specified using identifier or string quoting characters:

mysql> SELECT 1 AS `one`, 2 AS 'two';
+-----+-----+
| one | two |
+-----+-----+
|   1 |   2 |
+-----+-----+

Identifier quoting was introduced in MySQL 3.23.6 to permit use of identifiers that contain special characters or are reserved words. Before 3.23.6, you cannot use identifiers that require quotation marks, so the only legal characters are the set of alphanumeric characters from the current character set, _, and $.

Elsewhere in the statement, quoted references to the alias must use identifier quoting or the reference is treated as a string literal.

Identifiers may begin with a digit but unless quoted may not consist solely of digits.

It is recommended that you do not use names that begin with Me or MeN, where M and N are integers. For example, avoid using 1e as an identifier, because an expression such as 1e+3 is ambiguous. Depending on context, it might be interpreted as the expression 1e + 3 or as the number 1e+3.

Be careful when using MD5() to produce table names because it can produce names in illegal or ambiguous formats such as those just described.

A user variable cannot be used directly in an SQL statement as an identifier or as part of an identifier. See Section 8.4, “User-Defined Variables”, for more information and examples of workarounds.

There are some restrictions on the characters that may appear in identifiers:

  • No identifier can contain ASCII NUL (0x00) or a byte with a value of 255.

  • Before MySQL 4.1, identifier quote characters should not be used in identifiers.

  • Database, table, and column names should not end with space characters.

  • Database and table names cannot contain /, \, ., or characters that are not permitted in file names.

The following table describes the maximum length for each type of identifier. Before MySQL 4.1.5, the maximum-length restrictions on identifiers are measured in bytes, not characters. Until that version, if you use multi-byte characters in your identifier names, the maximum length will depend on the byte count of all the characters used.

IdentifierMaximum Length
Database64
Table64
Column64
Index64
Constraint64
Alias256

Beginning with MySQL 4.1, identifiers are stored using Unicode (UTF-8). This applies to identifiers in table definitions that are stored in .frm files and to identifiers stored in the grant tables in the mysql database. The sizes of the identifier string columns in the grant tables are measured in characters. You can use multi-byte characters without reducing the number of characters permitted for values stored in these columns, something not true prior to MySQL 4.1. The permissible Unicode characters are those in the Basic Multilingual Plane (BMP). Supplementary characters are not permitted.

8.2.1 Identifier Qualifiers

MySQL permits names that consist of a single identifier or multiple identifiers. The components of a multiple-part name must be separated by period (.) characters. The initial parts of a multiple-part name act as qualifiers that affect the context within which the final identifier is interpreted.

In MySQL, you can refer to a table column using any of the following forms.

Column ReferenceMeaning
col_nameThe column col_name from whichever table used in the statement contains a column of that name.
tbl_name.col_nameThe column col_name from table tbl_name of the default database.
db_name.tbl_name.col_nameThe column col_name from table tbl_name of the database db_name. This syntax is unavailable before MySQL 3.22.

If any components of a multiple-part name require quoting, quote them individually rather than quoting the name as a whole. For example, write `my-table`.`my-column`, not `my-table.my-column`.

A reserved word that follows a period in a qualified name must be an identifier, so in that context it need not be quoted.

You need not specify a tbl_name or db_name.tbl_name prefix for a column reference in a statement unless the reference would be ambiguous. Suppose that tables t1 and t2 each contain a column c, and you retrieve c in a SELECT statement that uses both t1 and t2. In this case, c is ambiguous because it is not unique among the tables used in the statement. You must qualify it with a table name as t1.c or t2.c to indicate which table you mean. Similarly, to retrieve from a table t in database db1 and from a table t in database db2 in the same statement, you must refer to columns in those tables as db1.t.col_name and db2.t.col_name.

The syntax .tbl_name means the table tbl_name in the default database. This syntax is accepted for ODBC compatibility because some ODBC programs prefix table names with a . character.

8.2.2 Identifier Case Sensitivity

In MySQL, databases correspond to directories within the data directory. Each table within a database corresponds to at least one file within the database directory (and possibly more, depending on the storage engine). Consequently, the case sensitivity of the underlying operating system plays a part in the case sensitivity of database and table names. This means database and table names are not case sensitive in Windows, and case sensitive in most varieties of Unix. One notable exception is Mac OS X, which is Unix-based but uses a default file system type (HFS+) that is not case sensitive. However, Mac OS X also supports UFS volumes, which are case sensitive just as on any Unix. See Section 1.9.4, “MySQL Extensions to Standard SQL”. The lower_case_table_names system variable also affects how the server handles identifier case sensitivity, as described later in this section.

Note

Although database and table names are not case sensitive on some platforms, you should not refer to a given database or table using different cases within the same statement. The following statement would not work because it refers to a table both as my_table and as MY_TABLE:

mysql> SELECT * FROM my_table WHERE MY_TABLE.col=1;

Column and index names are not case sensitive on any platform, nor are column aliases.

Table aliases are case sensitive before MySQL 4.1.1. The following query would not work because it refers to the alias both as a and as A:

mysql> SELECT col_name FROM tbl_name AS a
    -> WHERE a.col_name = 1 OR A.col_name = 2;

If you have trouble remembering the permissible lettercase for database and table names, it is best to adopt a consistent convention, such as always creating and referring to databases and tables using lowercase names. This convention is recommended for maximum portability and ease of use.

How table and database names are stored on disk and used in MySQL is affected by the lower_case_table_names system variable, which you can set when starting mysqld. lower_case_table_names can take the values shown in the following table. On Unix, the default value of lower_case_table_names is 0. On Windows, the default value is 1. On Mac OS X, the default is 1 before MySQL 4.0.18 and 2 as of 4.0.18.

ValueMeaning
0Table and database names are stored on disk using the lettercase specified in the CREATE TABLE or CREATE DATABASE statement. Name comparisons are case sensitive. You should not set this variable to 0 if you are running MySQL on a system that has case-insensitive file names (such as Windows or Mac OS X). If you force this variable to 0 with --lower-case-table-names=0 on a case-insensitive file system and access MyISAM tablenames using different lettercases, this may lead to index corruption.
1Table names are stored in lowercase on disk and name comparisons are not case sensitive. MySQL converts all table names to lowercase on storage and lookup. This behavior also applies to database names as of MySQL 4.0.2, and to table aliases as of 4.1.1.
2Table and database names are stored on disk using the lettercase specified in the CREATE TABLE or CREATE DATABASE statement, but MySQL converts them to lowercase on lookup. Name comparisons are not case sensitive. This works only on file systems that are not case sensitive! InnoDB table names are stored in lowercase, as for lower_case_table_names=1. Setting lower_case_table_names to 2 can be done as of MySQL 4.0.18.

If you are using MySQL on only one platform, you do not normally have to change the lower_case_table_names variable from its default value. However, you may encounter difficulties if you want to transfer tables between platforms that differ in file system case sensitivity. For example, on Unix, you can have two different tables named my_table and MY_TABLE, but on Windows those names are considered identical. To avoid data transfer problems arising from lettercase of database or table names, you have two options:

  • Use lower_case_table_names=1 on all systems. The main disadvantage with this is that when you use SHOW TABLES or SHOW DATABASES, you do not see the names in their original lettercase.

  • Use lower_case_table_names=0 on Unix and lower_case_table_names=2 on Windows. This preserves the lettercase of database and table names. The disadvantage of this is that you must ensure that your statements always refer to your database and table names with the correct lettercase on Windows. If you transfer your statements to Unix, where lettercase is significant, they do not work if the lettercase is incorrect.

    Exception: If you are using InnoDB tables and you are trying to avoid these data transfer problems, you should set lower_case_table_names to 1 on all platforms to force names to be converted to lowercase.

If you plan to set the lower_case_table_names system variable to 1 on Unix, you must first convert your old database and table names to lowercase before stopping mysqld and restarting it with the new variable setting.

8.2.3 Function Name Parsing and Resolution

MySQL 4.1 supports built-in (native) functions and user-defined functions (UDFs). This section describes how the server recognizes whether the name of a built-in function is used as a function call or as an identifier, and how the server determines which function to use in cases when functions of different types exist with a given name.

Built-In Function Name Parsing

The parser uses default rules for parsing names of built-in functions. These rules can be changed by enabling the IGNORE_SPACE SQL mode.

When the parser encounters a word that is the name of a built-in function, it must determine whether the name signifies a function call or is instead a nonexpression reference to an identifier such as a table or column name. For example, in the following statements, the first reference to count is a function call, whereas the second reference is a table name:

SELECT COUNT(*) FROM mytable;
CREATE TABLE count (i INT);

The parser should recognize the name of a built-in function as indicating a function call only when parsing what is expected to be an expression. That is, in nonexpression context, function names are permitted as identifiers.

However, some built-in functions have special parsing or implementation considerations, so the parser uses the following rules by default to distinguish whether their names are being used as function calls or as identifiers in nonexpression context:

  • To use the name as a function call in an expression, there must be no whitespace between the name and the following ( parenthesis character.

  • Conversely, to use the function name as an identifier, it must not be followed immediately by a parenthesis.

The requirement that function calls be written with no whitespace between the name and the parenthesis applies only to the built-in functions that have special considerations. COUNT is one such name. The exact list of function names for which following whitespace determines their interpretation are those listed in the sql_functions[] array of the sql/lex.h source file. Before MySQL 5.1, they are rather numerous (about 200), so you may find it easiest to treat the no-whitespace requirement as applying to all function calls. In MySQL 5.1, parser improvements reduce to about 30 the number of affected function names.

For functions not listed in the sql_functions[]) array, whitespace does not matter. They are interpreted as function calls only when used in expression context and may be used freely as identifiers otherwise. ASCII is one such name. However, for these nonaffected function names, interpretation may vary in expression context: func_name () is interpreted as a built-in function if there is one with the given name; if not, func_name () is interpreted as a user-defined function if one exists with that name.

The IGNORE_SPACE SQL mode can be used to modify how the parser treats function names that are whitespace-sensitive:

  • With IGNORE_SPACE disabled, the parser interprets the name as a function call when there is no whitespace between the name and the following parenthesis. This occurs even when the function name is used in nonexpression context:

    mysql> CREATE TABLE count(i INT);
    ERROR 1064 (42000): You have an error in your SQL syntax ...
    near 'count(i INT)'
    

    To eliminate the error and cause the name to be treated as an identifier, either use whitespace following the name or write it as a quoted identifier (or both):

    CREATE TABLE count (i INT);
    CREATE TABLE `count`(i INT);
    CREATE TABLE `count` (i INT);
    
  • With IGNORE_SPACE enabled, the parser loosens the requirement that there be no whitespace between the function name and the following parenthesis. This provides more flexibility in writing function calls. For example, either of the following function calls are legal:

    SELECT COUNT(*) FROM mytable;
    SELECT COUNT (*) FROM mytable;
    

    However, enabling IGNORE_SPACE also has the side effect that the parser treats the affected function names as reserved words (see Section 8.3, “Reserved Words”). This means that a space following the name no longer signifies its use as an identifier. The name can be used in function calls with or without following whitespace, but causes a syntax error in nonexpression context unless it is quoted. For example, with IGNORE_SPACE enabled, both of the following statements fail with a syntax error because the parser interprets count as a reserved word:

    CREATE TABLE count(i INT);
    CREATE TABLE count (i INT);
    

    To use the function name in nonexpression context, write it as a quoted identifier:

    CREATE TABLE `count`(i INT);
    CREATE TABLE `count` (i INT);
    

To enable the IGNORE_SPACE SQL mode, use this statement:

SET sql_mode = 'IGNORE_SPACE';

IGNORE_SPACE is also enabled by certain other composite modes such as ANSI that include it in their value:

SET sql_mode = 'ANSI';

Check Section 5.1.6, “Server SQL Modes”, to see which composite modes enable IGNORE_SPACE.

To minimize the dependency of SQL code on the IGNORE_SPACE setting, use these guidelines:

  • Avoid creating UDFs that have the same name as a built-in function.

  • Avoid using function names in nonexpression context. For example, these statements use count (one of the affected function names affected by IGNORE_SPACE), so they fail with or without whitespace following the name if IGNORE_SPACE is enabled:

    CREATE TABLE count(i INT);
    CREATE TABLE count (i INT);
    

    If you must use a function name in nonexpression context, write it as a quoted identifier:

    CREATE TABLE `count`(i INT);
    CREATE TABLE `count` (i INT);
    

Function Name Resolution

The server resolves references to function names for function creation and invocation as follows: A UDF can be created with the same name as a built-in function but the UDF cannot be invoked because the parser resolves invocations of the function to refer to the built-in function. For example, if you create a UDF named ABS, references to ABS() invoke the built-in function.

The preceding function name resolution rules have implications for upgrading to versions of MySQL that implement new built-in functions. If you have already created a user-defined function with a given name and upgrade MySQL to a version that implements a new built-in function with the same name, the UDF becomes inaccessible. To correct this, use DROP FUNCTION to drop the UDF, and then use CREATE FUNCTION to re-create the UDF with a different nonconflicting name.

8.3 Reserved Words

Certain words such as SELECT, DELETE, or BIGINT are reserved and require special treatment for use as identifiers such as table and column names. This may also be true for the names of built-in functions.

Reserved words are permitted as identifiers if you quote them as described in Section 8.2, “Database, Table, Index, Column, and Alias Names”:

mysql> CREATE TABLE interval (begin INT, end INT);
ERROR 1064 (42000): You have an error in your SQL syntax ...
near 'interval (begin INT, end INT)'

mysql> CREATE TABLE `interval` (begin INT, end INT);
Query OK, 0 rows affected (0.01 sec)

Exception: A word that follows a period in a qualified name must be an identifier, so it need not be quoted even if it is reserved:

mysql> CREATE TABLE mydb.interval (begin INT, end INT);
Query OK, 0 rows affected (0.01 sec)

Names of built-in functions are permitted as identifiers but may require care to be used as such. For example, COUNT is acceptable as a column name. However, by default, no whitespace is permitted in function invocations between the function name and the following ( character. This requirement enables the parser to distinguish whether the name is used in a function call or in nonfunction context. For further detail on recognition of function names, see Section 8.2.3, “Function Name Parsing and Resolution”.

The words in the following table are explicitly reserved in MySQL 4.1. At some point, you might upgrade to a higher version, so it is a good idea to have a look at future reserved words, too. You can find these in the manuals that cover higher versions of MySQL. Most of the words in the table are forbidden by standard SQL as column or table names (for example, GROUP). A few are reserved because MySQL needs them and uses a yacc parser. A reserved word can be used as an identifier if you quote it.

ADDALLALTER
ANALYZEANDAS
ASCBEFOREBETWEEN
BIGINTBINARYBLOB
BOTHBYCASCADE
CASECHANGECHAR
CHARACTERCHECKCOLLATE
COLUMNCOLUMNSCONSTRAINT
CONVERTCREATECROSS
CURRENT_DATECURRENT_TIMECURRENT_TIMESTAMP
CURRENT_USERDATABASEDATABASES
DAY_HOURDAY_MICROSECONDDAY_MINUTE
DAY_SECONDDECDECIMAL
DEFAULTDELAYEDDELETE
DESCDESCRIBEDISTINCT
DISTINCTROWDIVDOUBLE
DROPDUALELSE
ENCLOSEDESCAPEDEXISTS
EXPLAINFALSEFIELDS
FLOATFLOAT4FLOAT8
FORFORCEFOREIGN
FROMFULLTEXTGRANT
GROUPHAVINGHIGH_PRIORITY
HOUR_MICROSECONDHOUR_MINUTEHOUR_SECOND
IFIGNOREIN
INDEXINFILEINNER
INSERTINTINT1
INT2INT3INT4
INT8INTEGERINTERVAL
INTOISJOIN
KEYKEYSKILL
LEADINGLEFTLIKE
LIMITLINESLOAD
LOCALTIMELOCALTIMESTAMPLOCK
LONGLONGBLOBLONGTEXT
LOW_PRIORITYMATCHMEDIUMBLOB
MEDIUMINTMEDIUMTEXTMIDDLEINT
MINUTE_MICROSECONDMINUTE_SECONDMOD
NATURALNOTNO_WRITE_TO_BINLOG
NULLNUMERICON
OPTIMIZEOPTIONOPTIONALLY
ORORDEROUTER
OUTFILEPRECISIONPRIMARY
PRIVILEGESPROCEDUREPURGE
READREALREFERENCES
REGEXPRENAMEREPLACE
REQUIRERESTRICTREVOKE
RIGHTRLIKESECOND_MICROSECOND
SELECTSEPARATORSET
SHOWSMALLINTSONAME
SPATIALSQL_BIG_RESULTSQL_CALC_FOUND_ROWS
SQL_SMALL_RESULTSSLSTARTING
STRAIGHT_JOINTABLETABLES
TERMINATEDTHENTINYBLOB
TINYINTTINYTEXTTO
TRAILINGTRUEUNION
UNIQUEUNLOCKUNSIGNED
UPDATEUSAGEUSE
USINGUTC_DATEUTC_TIME
UTC_TIMESTAMPVALUESVARBINARY
VARCHARVARCHARACTERVARYING
WHENWHEREWITH
WRITEXORYEAR_MONTH
ZEROFILL  

The following are new reserved words in MySQL 4.0:

CHECKFORCELOCALTIME
LOCALTIMESTAMPREQUIRESQL_CALC_FOUND_ROWS
SSLXOR 

The following are new reserved words in MySQL 4.1:

BEFORECOLLATECONVERT
CURRENT_USERDAY_MICROSECONDDIV
DUALFALSEHOUR_MICROSECOND
MINUTE_MICROSECONDMODNO_WRITE_TO_BINLOG
SECOND_MICROSECONDSEPARATORSPATIAL
TRUEUTC_DATEUTC_TIME
UTC_TIMESTAMPVARCHARACTER 

MySQL permits some keywords to be used as unquoted identifiers because many people previously used them. Examples are those in the following list:

8.4 User-Defined Variables

MySQL supports user variables as of version 3.23.6. You can store a value in a user-defined variable in one statement and then refer to it later in another statement. This enables you to pass values from one statement to another. User-defined variables are connection-specific. That is, a user variable defined by one client cannot be seen or used by other clients. All variables for a given client connection are automatically freed when that client exits.

User variables are written as @var_name, where the variable name var_name consists of alphanumeric characters from the current character set, ., _, and $. A user variable name can contain other characters if you quote it as a string or identifier (for example, @'my-var', @"my-var", or @`my-var`). The default character set is latin1 (cp1252 West European). This can be changed with the --default-character-set option to mysqld. See Section 9.6, “Character Set Configuration”.

User variable names are not case sensitive in MySQL 5.0 and up, but are case sensitive before MySQL 5.0.

One way to set a user-defined variable is by issuing a SET statement:

SET @var_name = expr [, @var_name = expr] ...

For SET, either = or := can be used as the assignment operator.

You can also assign a value to a user variable in statements other than SET. In this case, the assignment operator must be := and not = because the latter is treated as the comparison operator = in non-SET statements:

mysql> SET @t1=1, @t2=2, @t3:=4;
mysql> SELECT @t1, @t2, @t3, @t4 := @t1+@t2+@t3;
+------+------+------+--------------------+
| @t1  | @t2  | @t3  | @t4 := @t1+@t2+@t3 |
+------+------+------+--------------------+
|    1 |    2 |    4 |                  7 | 
+------+------+------+--------------------+

User variables can be assigned a value from a limited set of data types: integer, decimal, floating-point, binary or nonbinary string, or NULL value. Assignment of decimal and real values does not preserve the precision or scale of the value. A value of a type other than one of the permissible types is converted to a permissible type. For example, a value having a temporal or spatial data type is converted to a binary string.

Beginning with MySQL 4.1.1, if a user variable is assigned a nonbinary (character) string value, it has the same character set and collation as the string. The coercibility of user variables is implicit as of MySQL 4.1.11 and 5.0.3. (This is the same coercibility as table column values.)

If the value of a user variable is selected in a result set, it is returned to the client as a string.

If you refer to a variable that has not been initialized, it has a value of NULL and a type of string.

User variables may be used in most contexts where expressions are permitted. This does not currently include contexts that explicitly require a literal value, such as in the LIMIT clause of a SELECT statement, or the IGNORE N LINES clause of a LOAD DATA statement.

As a general rule, you should never assign a value to a user variable and read the value within the same statement. You might get the results you expect, but this is not guaranteed. The order of evaluation for expressions involving user variables is undefined and may change based on the elements contained within a given statement. In SELECT @a, @a:=@a+1, ..., you might think that MySQL will evaluate @a first and then do an assignment second. However, changing the statement (for example, by adding a GROUP BY, HAVING, or ORDER BY clause) may cause MySQL to select an execution plan with a different order of evaluation.

Another issue with assigning a value to a variable and reading the value within the same statement is that the default result type of a variable is based on its type at the start of the statement. The following example illustrates this:

mysql> SET @a='test';
mysql> SELECT @a,(@a:=20) FROM tbl_name;

For this SELECT statement, MySQL reports to the client that column one is a string and converts all accesses of @a to strings, even though @a is set to a number for the second row. After the SELECT statement executes, @a is regarded as a number for the next statement.

To avoid problems with this behavior, either do not assign a value to and read the value of the same variable within a single statement, or else set the variable to 0, 0.0, or '' to define its type before you use it.

In a SELECT statement, each select expression is evaluated only when sent to the client. This means that in a HAVING, GROUP BY, or ORDER BY clause, referring to a variable that is assigned a value in the select expression list does not work as expected:

mysql> SELECT (@aa:=id) AS a, (@aa+3) AS b FROM tbl_name HAVING b=5;

The reference to b in the HAVING clause refers to an alias for an expression in the select list that uses @aa. This does not work as expected: @aa contains the value of id from the previous selected row, not from the current row.

User variables are intended to provide data values. They cannot be used directly in an SQL statement as an identifier or as part of an identifier, such as in contexts where a table or database name is expected, or as a reserved word such as SELECT. This is true even if the variable is quoted, as shown in the following example:

mysql> SELECT c1 FROM t;
+----+
| c1 |
+----+
|  0 |
+----+
|  1 |
+----+
2 rows in set (0.00 sec)

mysql> SET @col = "c1";
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT @col FROM t;
+------+
| @col |
+------+
| c1   |
+------+
1 row in set (0.00 sec)

mysql> SELECT `@col` FROM t;
ERROR 1054 (42S22): Unknown column '@col' in 'field list'

mysql> SET @col = "`c1`";
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT @col FROM t;
+------+
| @col |
+------+
| `c1` |
+------+
1 row in set (0.00 sec)

An exception to this principle that user variables cannot be used to provide identifiers is that if you are constructing a string for use as a prepared statement to be executed later. In this case, user variables can be used to provide any part of the statement. The following example illustrates how this can be done:

mysql> SET @c = "c1";
Query OK, 0 rows affected (0.00 sec)

mysql> SET @s = CONCAT("SELECT ", @c, " FROM t");
Query OK, 0 rows affected (0.00 sec)

mysql> PREPARE stmt FROM @s;
Query OK, 0 rows affected (0.04 sec)
Statement prepared

mysql> EXECUTE stmt;
+----+
| c1 |
+----+
|  0 |
+----+
|  1 |
+----+
2 rows in set (0.00 sec)

mysql> DEALLOCATE PREPARE stmt;
Query OK, 0 rows affected (0.00 sec)

See Section 12.6, “SQL Syntax for Prepared Statements”, for more information.

PREPARE is available as of MySQL 4.1. Before MySQL 4.1, you must assemble a string for the query in application code, as shown here using PHP 5:

<?php
  $mysqli = new mysqli("localhost", "user", "pass", "test");

  if( mysqli_connect_errno() )
    die("Connection failed: %s\n", mysqli_connect_error());

  $col = "c1";

  $query = "SELECT $col FROM t";

  $result = $mysqli->query($query);

  while($row = $result->fetch_assoc())
  {
    echo "<p>" . $row["$col"] . "</p>\n";
  }

  $result->close();

  $mysqli->close();
?>

Assembling an SQL statement in this fashion is sometimes known as Dynamic SQL.

8.5 Expression Syntax

The following rules define expression syntax in MySQL. The grammar shown here is based on that given in the sql/sql_yacc.yy file of MySQL source distributions. See the notes after the grammar for additional information about some of the terms. Operator precedence is given in Section 11.3.1, “Operator Precedence”.

expr:
    expr OR expr
  | expr || expr
  | expr XOR expr
  | expr AND expr
  | expr && expr
  | NOT expr
  | ! expr
  | boolean_primary IS [NOT] {TRUE | FALSE | UNKNOWN}
  | boolean_primary

boolean_primary:
    boolean_primary IS [NOT] NULL
  | boolean_primary <=> predicate
  | boolean_primary comparison_operator predicate
  | boolean_primary comparison_operator {ALL | ANY} (subquery)
  | predicate

comparison_operator: = | >= | > | <= | < | <> | !=

predicate:
    bit_expr [NOT] IN (subquery)
  | bit_expr [NOT] IN (expr [, expr] ...)
  | bit_expr [NOT] BETWEEN bit_expr AND predicate
  | bit_expr SOUNDS LIKE bit_expr
  | bit_expr [NOT] LIKE simple_expr [ESCAPE simple_expr]
  | bit_expr [NOT] REGEXP bit_expr
  | bit_expr

bit_expr:
    bit_expr | bit_expr
  | bit_expr & bit_expr
  | bit_expr << bit_expr
  | bit_expr >> bit_expr
  | bit_expr + bit_expr
  | bit_expr - bit_expr
  | bit_expr * bit_expr
  | bit_expr / bit_expr
  | bit_expr DIV bit_expr
  | bit_expr MOD bit_expr
  | bit_expr % bit_expr
  | bit_expr ^ bit_expr
  | bit_expr + interval_expr
  | bit_expr - interval_expr
  | simple_expr

simple_expr:
    literal
  | identifier
  | function_call
  | simple_expr COLLATE collation_name
  | param_marker
  | variable
  | simple_expr || simple_expr
  | + simple_expr
  | - simple_expr
  | ~ simple_expr
  | ! simple_expr
  | BINARY simple_expr
  | (expr [, expr] ...)
  | ROW (expr, expr [, expr] ...)
  | (subquery)
  | EXISTS (subquery)
  | {identifier expr}
  | match_expr
  | case_expr
  | interval_expr

Notes:

For literal value syntax, see Section 8.1, “Literal Values”.

For identifier syntax, see Section 8.2, “Database, Table, Index, Column, and Alias Names”.

Variables can be user variables, system variables, or stored program local variables or parameters:

param_marker is '?' as used in prepared statements for placeholders. See Section 12.6.1, “PREPARE Syntax”.

(subquery) indicates a subquery that returns a single value; that is, a scalar subquery. See Section 12.2.8.1, “The Subquery as Scalar Operand”.

{identifier expr} is ODBC escape syntax and is accepted for ODBC compatibility. The value is expr. The curly braces in the syntax should be written literally; they are not metasyntax as used elsewhere in syntax descriptions.

match_expr indicates a MATCH expression. See Section 11.9, “Full-Text Search Functions”.

case_expr indicates a CASE expression. See Section 11.4, “Control Flow Functions”.

interval_expr represents a time interval. The syntax is INTERVAL expr unit, where unit is a specifier such as HOUR, DAY, or WEEK. For the full list of unit specifiers, see the description of the DATE_ADD() function in Section 11.7, “Date and Time Functions”.

The meaning of the || operator depends on the SQL mode. By default, || is a logical OR operator. With PIPES_AS_CONCAT enabled, || is string concatenation, with a precedence between ^ and the unary operators. See Section 5.1.6, “Server SQL Modes”.

8.6 Comment Syntax

MySQL Server supports three comment styles:

  • From a # character to the end of the line.

  • From a --  sequence to the end of the line. This style is supported as of MySQL 3.23.3. In MySQL, the --  (double-dash) comment style requires the second dash to be followed by at least one whitespace or control character (such as a space, tab, newline, and so on). This syntax differs slightly from standard SQL comment syntax, as discussed in Section 1.9.5.8, “'--' as the Start of a Comment”.

  • From a /* sequence to the following */ sequence, as in the C programming language. This syntax enables a comment to extend over multiple lines because the beginning and closing sequences need not be on the same line.

The following example demonstrates all three comment styles:

mysql> SELECT 1+1;     # This comment continues to the end of line
mysql> SELECT 1+1;     -- This comment continues to the end of line
mysql> SELECT 1 /* this is an in-line comment */ + 1;
mysql> SELECT 1+
/*
this is a
multiple-line comment
*/
1;

Nested comments are not supported.

MySQL Server supports some variants of C-style comments. These enable you to write code that includes MySQL extensions, but is still portable, by using comments of the following form:

/*! MySQL-specific code */

In this case, MySQL Server parses and executes the code within the comment as it would any other SQL statement, but other SQL servers will ignore the extensions. For example, MySQL Server recognizes the STRAIGHT_JOIN keyword in the following statement, but other servers will not:

SELECT /*! STRAIGHT_JOIN */ col1 FROM table1,table2 WHERE ...

If you add a version number after the ! character, the syntax within the comment is executed only if the MySQL version is greater than or equal to the specified version number. The TEMPORARY keyword in the following comment is executed only by servers from MySQL 3.23.02 or higher:

CREATE /*!32302 TEMPORARY */ TABLE t (a INT);

The comment syntax just described applies to how the mysqld server parses SQL statements. The mysql client program also performs some parsing of statements before sending them to the server. (It does this to determine statement boundaries within a multiple-statement input line.) However, there are some limitations on the way that mysql parses /* ... */ comments:

  • A semicolon within the comment is taken to indicate the end of the current SQL statement and anything following it to indicate the beginning of the next statement. This problem was fixed in MySQL 4.0.13.

  • A single quote, double quote, or backtick character is taken to indicate the beginning of a quoted string or identifier, even within a comment. If the quote is not matched by a second quote within the comment, the parser doesn't realize the comment has ended. If you are running mysql interactively, you can tell that it has gotten confused like this because the prompt changes from mysql> to '>, ">, or `>. This problem was fixed in MySQL 4.1.1.

  • The use of short-form commands such as \C within multi-line /* ... */ comments is not supported.

Comments in this format, /*!12345 ... */, are not stored on the server. If this format is used to comment stored routines, the comments will not be retained on the server.

For affected versions of MySQL, these limitations apply both when you run mysql interactively and when you put commands in a file and use mysql in batch mode to process the file with mysql < file_name.