Table of Contents
This chapter discusses the rules for writing the following elements of SQL statements when using MySQL:
Literal values such as strings and numbers
Identifiers such as database, table, and column names
Reserved words
User-defined and system variables
Comments
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.
A string is a sequence of bytes or characters, enclosed within
either single quote (“'
”) or
double quote (“"
”) characters.
Examples:
'a string' "another string"
If the ANSI_QUOTES
SQL mode is enabled,
string literals can be quoted only within single quotes because
a string quoted within double quotes is interpreted as an
identifier.
A binary string is a string of bytes that has no character set or collation. A non-binary 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 non-binary strings the unit is the character and some character sets allow multi-byte characters. Character value ordering is a function of the string collation.
String literals may have an optional character set introducer
and COLLATE
clause:
[_charset_name
]'string
' [COLLATEcollation_name
]
Examples:
SELECT _latin1'string
'; SELECT _latin1'string
' COLLATE latin1_danish_ci;
You can use
N'
(or
literal
'n'
) to
create a string in the national character set. These statements
are equivalent:
literal
'
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:
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 11.4.1, “String Comparison Functions”. If you use
“\%
” or
“\_
” in non-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:
NUL | NUL byte (ASCII 0). 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 23.2.3.53, “mysql_real_escape_string()
”. The Perl DBI
interface provides a quote
method to
convert special characters to the proper escape sequences.
See Section 23.4, “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.
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.
MySQL supports hexadecimal values. In numeric contexts, these act like integers (64-bit precision). In string contexts, these 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'
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'
syntax is based on standard SQL. The hexstring
'0x
syntax is based on ODBC. Hexadecimal strings are often used by
ODBC to supply values for BLOB
columns.
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'
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
Beginning with MySQL 5.0.3, bit-field values can be written
using b'
or
value
'0b
notation.
value
value
is a binary value written using
zeros and ones.
Bit-field notation is convenient for specifying values to be
assigned to BIT
columns:
mysql>CREATE TABLE t (b BIT(8));
mysql>INSERT INTO t SET b = b'11111111';
mysql>INSERT INTO t SET b = b'1010';
mysql>INSERT INTO t SET b = b'0101';
Bit values are returned as binary values. To display them in
printable form, add 0 or use a conversion function such as
BIN()
. High-order 0 bits are not
displayed in the converted value.
mysql> SELECT b+0, BIN(b+0), OCT(b+0), HEX(b+0) FROM t;
+------+----------+----------+----------+
| b+0 | BIN(b+0) | OCT(b+0) | HEX(b+0) |
+------+----------+----------+----------+
| 255 | 11111111 | 377 | FF |
| 10 | 1010 | 12 | A |
| 5 | 101 | 5 | 5 |
+------+----------+----------+----------+
Bit values assigned to user variables are treated as binary
strings. To assign a bit value as a number to a user variable,
use CAST()
or
+0
:
mysql>SET @v1 = b'1000001';
mysql>SET @v2 = CAST(b'1000001' AS UNSIGNED), @v3 = b'1000001'+0;
mysql>SELECT @v1, @v2, @v3;
+------+------+------+ | @v1 | @v2 | @v3 | +------+------+------+ | A | 65 | 65 | +------+------+------+
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.1.5.3, “Problems with NULL
Values”.
Database, table, index, column, and alias names are identifiers. This section describes the allowable 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 allowable to quote identifiers within double quotes:
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 quotes. They cannot be enclosed within double quotes.
The server SQL mode is controlled as described in Section 5.1.6, “SQL Modes”.
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, then you need to 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);
Identifiers may begin with a digit but unless quoted may not consist solely of digits.
It is recommended that you do not use names of the form
or
M
e
,
where M
eN
M
and
N
are integers. For example, avoid
using 1e
or 2e2
as
identifiers, 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 0 (0x00
) or
a byte with a value of 255.
Database, table, and column names should not end with space characters.
Database and table names cannot contain
“/
”,
“\
”,
“.
”, or characters that are
not allowed in filenames.
The following table describes the maximum length for each type of identifier.
Identifier | Maximum Length (characters) |
Database | 64 |
Table | 64 |
Column | 64 |
Index | 64 |
Stored Function or Procedure | 64 |
Trigger | 64 |
View | 64 |
Alias | 255 |
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 allowed for values stored in these
columns, something not true prior to MySQL 4.1. The allowable
Unicode characters are those in the Basic Multilingual Plane
(BMP). Supplementary characters are not allowed.
MySQL allows 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 Reference | Meaning |
col_name | The column col_name from whichever table used
in the statement contains a column of that name. |
tbl_name.col_name | The column col_name from table
tbl_name of the default
database. |
db_name.tbl_name.col_name | The column col_name from table
tbl_name of the database
db_name . |
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.
and
col_name
db2.t.
.
col_name
The syntax
means
the table .tbl_name
tbl_name
in the default
database. This syntax is accepted for ODBC compatibility because
some ODBC programs prefix table names with a
“.
” character.
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 filesystem
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 1.8.4, “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.
MySQL Enterprise
lower_case_table_names
is just one of the
system variables monitored by the MySQL Enterprise Monitor.
For information about subscribing to this service, see
http://www.mysql.com/products/enterprise/advisors.html.
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, index, and stored routine names are not case sensitive on any platform, nor are column aliases. Trigger names are case sensitive, which differs from standard SQL.
By default, table aliases are case sensitive on Unix, but not so
on Windows or Mac OS X. The following statement would not work
on Unix, because it refers to the alias both as
a
and as A
:
mysql>SELECT
->col_name
FROMtbl_name
AS aWHERE a.
col_name
= 1 OR A.col_name
= 2;
However, this same statement is permitted on Windows. To avoid problems caused by such differences, 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 value is 2.
Value | Meaning |
0 | Table and database names are stored on disk using the lettercase
specified in the CREATE TABLE or
CREATE DATABASE statement. Name
comparisons are case sensitive. Note that if you force
this variable to 0 with
--lower-case-table-names=0 on a
case-insensitive filesystem and access
MyISAM tablenames using different
lettercases, index corruption may result. |
1 | Table 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 and table aliases. |
2 | Table 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 filesystems that are not
case sensitive! InnoDB table names
are stored in lowercase, as for
lower_case_table_names=1 . |
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 filesystem case sensitivity. For example, on
Unix, you can have two different tables named
my_table
and MY_TABLE
, but
on Windows these two 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.
Object names may be considered duplicates if their uppercase
forms are equal according to a binary collation. That is true
for names of cursors, conditions, functions, procedures,
savepoints, and routine local variables. It is not true for
names of columns, constraints, databases, statements prepared
with PREPARE
, tables, triggers, users, and
user-defined variables.
MySQL 5.0 supports built-in (native) functions, user-defined functions (UDFs), and stored functions. 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 non-expression 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 non-expression 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 non-expression 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 non-affected function names, interpretation may vary
in expression context:
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 or stored function if one
exists with that name.
func_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
non-expression 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
non-expression 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 non-expression 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, “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 or stored functions that have the same name as a built-in function.
Avoid using function names in non-expression 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 non-expression context, write it as a quoted identifier:
CREATE TABLE `count`(i INT); CREATE TABLE `count` (i INT);
Function Name Resolution
The following rules describe how the server resolves references to function names for function creation and invocation:
Built-in functions and user-defined functions
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.
Built-in functions and stored functions
It is possible to create a stored function with the same
name as a built-in function, but to invoke the stored
function it is necessary to qualify it with a database name.
For example, if you create a stored function named
PI
in the test
database, you invoke it as test.PI()
because the server resolves
PI()
as a reference to the
built-in function.
User-defined functions and stored functions
User-defined functions and stored functions share the same namespace, so you cannot create a UDF and a stored function with the same name.
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 non-conflicting name.
If a new version of MySQL implements a built-in function
with the same name as an existing stored function, you have
two choices: Rename the stored function to use a
non-conflicting name, or change calls to the function so
that they use a schema qualifier (that is, use
syntax).
schema_name
.func_name
()
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, “Schema Object 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 allowed 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 non-function 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
5.0. At some point, you might upgrade to a higher
version, so it's 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.
For a more detailed list of reserved words, including differences between versions, see Reserved Words in MySQL 5.0.
ADD | ALL | ALTER |
ANALYZE | AND | AS |
ASC | ASENSITIVE | BEFORE |
BETWEEN | BIGINT | BINARY |
BLOB | BOTH | BY |
CALL | CASCADE | CASE |
CHANGE | CHAR | CHARACTER |
CHECK | COLLATE | COLUMN |
CONDITION | CONSTRAINT | CONTINUE |
CONVERT | CREATE | CROSS |
CURRENT_DATE | CURRENT_TIME | CURRENT_TIMESTAMP |
CURRENT_USER | CURSOR | DATABASE |
DATABASES | DAY_HOUR | DAY_MICROSECOND |
DAY_MINUTE | DAY_SECOND | DEC |
DECIMAL | DECLARE | DEFAULT |
DELAYED | DELETE | DESC |
DESCRIBE | DETERMINISTIC | DISTINCT |
DISTINCTROW | DIV | DOUBLE |
DROP | DUAL | EACH |
ELSE | ELSEIF | ENCLOSED |
ESCAPED | EXISTS | EXIT |
EXPLAIN | FALSE | FETCH |
FLOAT | FLOAT4 | FLOAT8 |
FOR | FORCE | FOREIGN |
FROM | FULLTEXT | GRANT |
GROUP | HAVING | HIGH_PRIORITY |
HOUR_MICROSECOND | HOUR_MINUTE | HOUR_SECOND |
IF | IGNORE | IN |
INDEX | INFILE | INNER |
INOUT | INSENSITIVE | INSERT |
INT | INT1 | INT2 |
INT3 | INT4 | INT8 |
INTEGER | INTERVAL | INTO |
IS | ITERATE | JOIN |
KEY | KEYS | KILL |
LEADING | LEAVE | LEFT |
LIKE | LIMIT | LINES |
LOAD | LOCALTIME | LOCALTIMESTAMP |
LOCK | LONG | LONGBLOB |
LONGTEXT | LOOP | LOW_PRIORITY |
MATCH | MEDIUMBLOB | MEDIUMINT |
MEDIUMTEXT | MIDDLEINT | MINUTE_MICROSECOND |
MINUTE_SECOND | MOD | MODIFIES |
NATURAL | NOT | NO_WRITE_TO_BINLOG |
NULL | NUMERIC | ON |
OPTIMIZE | OPTION | OPTIONALLY |
OR | ORDER | OUT |
OUTER | OUTFILE | PRECISION |
PRIMARY | PROCEDURE | PURGE |
READ | READS | REAL |
REFERENCES | REGEXP | RELEASE |
RENAME | REPEAT | REPLACE |
REQUIRE | RESTRICT | RETURN |
REVOKE | RIGHT | RLIKE |
SCHEMA | SCHEMAS | SECOND_MICROSECOND |
SELECT | SENSITIVE | SEPARATOR |
SET | SHOW | SMALLINT |
SONAME | SPATIAL | SPECIFIC |
SQL | SQLEXCEPTION | SQLSTATE |
SQLWARNING | SQL_BIG_RESULT | SQL_CALC_FOUND_ROWS |
SQL_SMALL_RESULT | SSL | STARTING |
STRAIGHT_JOIN | TABLE | TERMINATED |
THEN | TINYBLOB | TINYINT |
TINYTEXT | TO | TRAILING |
TRIGGER | TRUE | UNDO |
UNION | UNIQUE | UNLOCK |
UNSIGNED | UPDATE | USAGE |
USE | USING | UTC_DATE |
UTC_TIME | UTC_TIMESTAMP | VALUES |
VARBINARY | VARCHAR | VARCHARACTER |
VARYING | WHEN | WHERE |
WHILE | WITH | WRITE |
XOR | YEAR_MONTH | ZEROFILL |
The following are new reserved words in MySQL 5.0:
ASENSITIVE | CALL | CONDITION |
CONNECTION | CONTINUE | CURSOR |
DECLARE | DETERMINISTIC | EACH |
ELSEIF | EXIT | FETCH |
GOTO | INOUT | INSENSITIVE |
ITERATE | LABEL | LEAVE |
LOOP | MODIFIES | OUT |
READS | RELEASE | REPEAT |
RETURN | SCHEMA | SCHEMAS |
SENSITIVE | SPECIFIC | SQL |
SQLEXCEPTION | SQLSTATE | SQLWARNING |
TRIGGER | UNDO | UPGRADE |
WHILE |
MySQL allows some keywords to be used as unquoted identifiers because many people previously used them. Examples are those in the following list:
ACTION
BIT
DATE
ENUM
NO
TEXT
TIME
TIMESTAMP
You can store a value in a user-defined variable and then refer to it later. 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
@
, where the
variable name var_name
var_name
may consist of
alphanumeric characters from the current character set,
“.
”,
“_
”, and
“$
”. The default character set is
latin1
(cp1252 West European). This may be
changed with the --default-character-set
option
to mysqld. See
Section 9.2, “The Character Set Used for Data and Sorting”. 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`
).
Note: User variable names are case sensitive before MySQL 5.0 and not case sensitive in MySQL 5.0 and up.
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. The
expr
assigned to each variable can
evaluate to an integer, decimal, floating-point, string, or
NULL
value. However, if the value of the
variable is selected in a result set, it is returned to the client
as a string. Assignment of decimal and real values does not
preserve the precision or scale of the value.
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
=
is treated as a comparison operator in
non-SET
statements:
mysql>SET @t1=0, @t2=0, @t3=0;
mysql>SELECT @t1:=(@t2:=1)+@t3:=4,@t1,@t2,@t3;
+----------------------+------+------+------+ | @t1:=(@t2:=1)+@t3:=4 | @t1 | @t2 | @t3 | +----------------------+------+------+------+ | 5 | 5 | 1 | 4 | +----------------------+------+------+------+
User variables may be used in contexts where expressions are
allowed. 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
clause of a N
LINESLOAD DATA
statement.
If a user variable is assigned a string value, it has the same character set and collation as the string. The coercibility of user variables is implicit as of MySQL 5.0.3. (This is the same coercibility as for table column values.)
If you refer to a variable that has not been initialized, it has a
value of NULL
and a type of string.
Bit values assigned to user variables are treated as binary
strings. To assign a bit value as a number to a user variable, use
CAST()
or +0
:
mysql>SET @v1 = b'1000001';
mysql>SET @v2 = CAST(b'1000001' AS UNSIGNED), @v3 = b'1000001'+0;
mysql>SELECT @v1, @v2, @v3;
+------+------+------+ | @v1 | @v2 | @v3 | +------+------+------+ | A | 65 | 65 | +------+------+------+
In a SELECT
statement, each expression is
evaluated only when sent to the client. This means that in a
HAVING
, GROUP BY
, or
ORDER BY
clause, you cannot refer to an
expression that involves variables that are set in the
SELECT
list. For example, the following
statement 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.
The order of evaluation for user variables is undefined and may
change based on the elements contained within a given query. In
SELECT @a, @a := @a+1 ...
, you might think that
MySQL will evaluate @a
first and then do an
assignment second, but changing the query (for example, by adding
a GROUP BY
, HAVING
, or
ORDER BY
clause) may change the order of
evaluation.
The general rule is never to assign a value to a user variable in one part of a statement and use the same variable in some other part of the same statement. You might get the results you expect, but this is not guaranteed.
Another issue with setting a variable and using it in the same statement is that the default result type of a variable is based on the type of the variable 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 set and use
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.
A user variable cannot be used directly in an SQL statement as an identifier or as part of an identifier, even if it is set off with backticks. This is 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)
One way to work around this problem is to 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”.) It is also possible to perform such operations using prepared statements, without the need to concatenate strings of SQL in client code. This 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)
You cannot use a placeholder for an identifier (such as the name of a database, table, or column) in an SQL prepared statement. See Section 12.7, “SQL Syntax for Prepared Statements”, for more information.
MySQL Server supports three comment styles:
From a “#
” character to the
end of the line.
From a “--
” sequence to
the end of the line. 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 1.8.5.6, “'--
' as the Start of a Comment”.
From a /*
sequence to the following
*/
sequence, as in the C programming
language. This syntax allows 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.)
The use of short-form mysql commands such as
\C
within multi-line /* ...
*/
comments is not supported.