要在表达式中将名称用作函数调用，名称与后面的 ( 括号字符之间不能有空格。To use the name as a function call in an expression, there must be no whitespace between the name and the following ( parenthesis character.

相反，要将函数名用作标识符，它后面不能紧跟着括号。Conversely, to use the function name as an identifier, it must not be followed immediately by a parenthesis.

ADDDATE

BIT_ANDBIT_AND

BIT_ORBIT_OR

BIT_XORBIT_XOR

CASTCAST

COUNTCOUNT

CURDATECURDATE

CURTIMECURTIME

DATE_ADDDATE_ADD

DATE_SUBDATE_SUB

EXTRACTEXTRACT

GROUP_CONCATGROUP_CONCAT

MAXMAX

MIDMID

MINMIN

NOW

POSITION

SESSION_USER

STD

STDDEV

STDDEV_POP

STDDEV_SAMP

SUBDATE

SUBSTR

SUBSTRING

SUM

SYSDATE

SYSTEM_USER

TRIMTRIM

VARIANCEVARIANCE

VAR_POPVAR_POP

VAR_SAMPVAR_SAMP

当 IGNORE_SPACE 被禁用时，当函数名与后续括号之间没有空格时，解析器会将该名称解释为函数调用。即使函数名在非表达式上下文中使用，也会发生这种情况：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)'

要消除错误并使名称被视为标识符，可以在名称后使用空格，或将其写为带引号的标识符（或两者兼用）：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);

启用 IGNORE_SPACE 后，解析器会放宽对函数名与随后的括号之间不能有空格的要求。这为编写函数调用提供了更多的灵活性。例如，以下任意一个函数调用都是合法的：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;

然而，启用 IGNORE_SPACE 也有副作用，即解析器将受影响的函数名称视为保留字（参见 第 11.3 节，“关键字和保留字”）。这意味着名称后跟空格不再表示其用作标识符。该名称可以在函数调用中使用，无论后面是否有空格，但在非表达式上下文中会导致语法错误，除非加上引号。例如，在启用 IGNORE_SPACE 的情况下，以下两个语句都会因语法错误而失败，因为解析器将 count 解释为保留字：However, enabling IGNORE_SPACE also has the side effect that the parser treats the affected function names as reserved words (see Section 11.3, “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);

要在非表达式上下文中使用函数名，请将其写为带引号的标识符：To use the function name in nonexpression context, write it as a quoted identifier:

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

避免创建与内置函数同名的可加载函数或存储函数。Avoid creating loadable functions or stored functions that have the same name as a built-in function.

避免在非表达式上下文中使用函数名称。例如，这些语句使用了count（这是受IGNORE_SPACE影响的函数名称之一），因此如果启用了IGNORE_SPACE，无论名称后是否有空格，它们都会失败：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);

如果必须在非表达式上下文中使用函数名，请将其写为带引号的标识符：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);

内置函数和可加载函数Built-in functions and loadable functions

如果您尝试创建一个与内置函数同名的可加载函数，将会发生错误。An error occurs if you try to create a loadable function with the same name as a built-in function.

IF NOT EXISTS 在这种情况下不起作用。更多信息请参见 第 15.7.4.1 节，“可加载函数的 CREATE FUNCTION 语句”。IF NOT EXISTS has no effect in such cases. See Section 15.7.4.1, “CREATE FUNCTION Statement for Loadable Functions”, for more information.

内置函数和存储函数Built-in functions and stored functions

可以创建与内置函数同名的存储函数，但要调用该存储函数，必须使用模式名称进行限定。例如，如果在 test 模式中创建一个名为 PI 的存储函数，则调用时应使用 test.PI()，因为服务器会将未限定的 PI() 解析为对内置函数的引用。如果存储函数名称与内置函数名称冲突，服务器会生成警告。这些警告可以通过 SHOW WARNINGS 来显示。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.

IF NOT EXISTS 在这种情况下没有效果；请参见 第 15.1.17 节，“CREATE PROCEDURE 和 CREATE FUNCTION 语句”。IF NOT EXISTS has no effect in such cases; see Section 15.1.17, “CREATE PROCEDURE and CREATE FUNCTION Statements”.

可加载函数和存储函数Loadable functions and stored functions

可以创建与现有可加载函数同名的存储函数，反之亦然。如果拟议的存储函数名称与现有可加载函数名称冲突，或者拟议的可加载函数名称与现有存储函数名称相同，服务器将生成警告。在任何一种情况下，一旦两个函数都存在，以后在调用存储函数时必须使用模式名称来限定它；在这种情况下，服务器假定未限定的名称指的是可加载函数。It is possible to create a stored function with the same name as an existing loadable function, or the other way around. The server generates a warning if a proposed stored function name collides with an existing loadable function name, or if a proposed loadable function name would be the same as that of an existing stored function. In either case, once both functions exist, it is necessary thereafter to qualify the stored function with a schema name when invoking it; the server assumes in such cases that the unqualified name refers to the loadable function.

MySQL 8.4 支持在 IF NOT EXISTS 语句中使用 CREATE FUNCTION，但在这种情况下没有效果。MySQL 8.4 supports IF NOT EXISTS with CREATE FUNCTION statements, but it has no effect in such cases.

如果您已经创建了具有给定名称的可加载函数，并且将 MySQL 升级到一个实现了具有相同名称的新内置函数的版本，则该可加载函数将无法访问。要纠正此情况，请使用 DROP FUNCTION 删除可加载函数，然后使用 CREATE FUNCTION 以不同的、不冲突的名称重新创建可加载函数。然后修改任何受影响的代码以使用新名称。If you have already created a loadable function with a given name and upgrade MySQL to a version that implements a new built-in function with the same name, the loadable function becomes inaccessible. To correct this, use DROP FUNCTION to drop the loadable function and CREATE FUNCTION to re-create the loadable function with a different nonconflicting name. Then modify any affected code to use the new name.

如果 MySQL 的新版本实现了一个与现有存储函数同名的内置函数或可加载函数，你有两个选择：将存储函数重命名为一个不冲突的名称，或者将所有尚未使用模式限定符调用的函数调用更改为使用模式限定符（schema_name.func_name() 语法）。在任何情况下，相应地修改受影响的代码。If a new version of MySQL implements a built-in function or loadable 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 any calls to the function that do not do so already to use a schema qualifier (schema_name.func_name() syntax). In either case, modify any affected code accordingly.