MySQL 5.6 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 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
sql_functions[] array in the
sql/lex.h source file lists the names of
these special functions for which following whitespace
determines their interpretation. In MySQL 5.6,
there are about 30 such function names. You may find it easiest
to treat the no-whitespace requirement as applying to all
function calls.
The following table names the functions that are affected by the
IGNORE_SPACE setting and
listed as special in the sql/lex.h source
file.
ADDDATE | BIT_AND | BIT_OR | BIT_XOR |
CAST | COUNT | CURDATE | CURTIME |
DATE_ADD | DATE_SUB | EXTRACT | GROUP_CONCAT |
MAX | MID | MIN | NOW |
POSITION | SESSION_USER | STD | STDDEV |
STDDEV_POP | STDDEV_SAMP | SUBDATE | SUBSTR |
SUBSTRING | SUM | SYSDATE | SYSTEM_USER |
TRIM | VARIANCE | VAR_POP | VAR_SAMP |
For functions not listed as special in
sql/lex.h, 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 ()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 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 9.3, “Keywords and 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 5.1.8, “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 or stored functions 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 following rules describe how the server resolves references to function names for function creation and invocation:
Built-in functions and user-defined functions
An error occurs if you try to create a UDF with the same name as a 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 schema name.
For example, if you create a stored function named
PI in the test schema,
invoke it as test.PI() because the server
resolves PI() without a
qualifier as a reference to the built-in function. The
server generates a warning if the stored function name
collides with a built-in function name. The warning can be
displayed with SHOW WARNINGS.
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
CREATE FUNCTION to re-create
the UDF with a different nonconflicting name. Then modify
any affected code to use the new 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
nonconflicting name, or change calls to the function so that
they use a schema qualifier (that is, use
schema_name.func_name()