### 9.2.5 Function Name Parsing and Resolution

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 MySQL supports built-in (native) functions, loadable functions, 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](function-resolution.html#function-name-parsing)

* [Function Name Resolution](function-resolution.html#function-name-resolution)

#### 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.html#sqlmode_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/lex.h` source file lists the names of these special functions for which following whitespace determines their interpretation: names defined by the `SYM_FN()` macro in the `symbols[]` array.

 The following list names the functions in MySQL 8.0 that are affected by the [`IGNORE_SPACE`](sql-mode.html#sqlmode_ignore_space) setting and listed as special in the `sql/lex.h` source file. You may find it easiest to treat the no-whitespace requirement as applying to all function calls.

* `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`* ()` is interpreted as a built-in function if there is one with the given name; if not, `*`func_name`* ()` is interpreted as a loadable function or stored function if one exists with that name.

 The [`IGNORE_SPACE`](sql-mode.html#sqlmode_ignore_space) SQL mode can be used to modify how the parser treats function names that are whitespace-sensitive:

* With [`IGNORE_SPACE`](sql-mode.html#sqlmode_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`](sql-mode.html#sqlmode_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`](sql-mode.html#sqlmode_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”](keywords.html)). 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`](sql-mode.html#sqlmode_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.html#sqlmode_ignore_space) SQL mode, use this statement:

```
SET sql_mode = 'IGNORE_SPACE';
```

[`IGNORE_SPACE`](sql-mode.html#sqlmode_ignore_space) is also enabled by certain other composite modes such as [`ANSI`](sql-mode.html#sqlmode_ansi) that include it in their value:

```
SET sql_mode = 'ANSI';
```

 Check [Section 5.1.11, “Server SQL Modes”](sql-mode.html), to see which composite modes enable [`IGNORE_SPACE`](sql-mode.html#sqlmode_ignore_space).

 To minimize the dependency of SQL code on the [`IGNORE_SPACE`](sql-mode.html#sqlmode_ignore_space) setting, use these guidelines:

* Avoid creating loadable functions 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`](sql-mode.html#sqlmode_ignore_space)), so they fail with or without whitespace following the name if [`IGNORE_SPACE`](sql-mode.html#sqlmode_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 loadable functions

   An error occurs if you try to create a loadable function with the same name as a built-in function.

  `IF NOT EXISTS` (available beginning with MySQL 8.0.29) has no effect in such cases. See [Section 13.7.4.1, “CREATE FUNCTION Statement for Loadable Functions”](create-function-loadable.html), for more information.

* 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()`](mathematical-functions.html#function_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`](show-warnings.html).

  `IF NOT EXISTS` (MySQL 8.0.29 and later) has no effect in such cases; see [Section 13.1.17, “CREATE PROCEDURE and CREATE FUNCTION Statements”](create-procedure.html).

* Loadable functions and stored functions

   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.

   Beginning with MySQL 8.0.29, `IF NOT EXISTS` is supported with [`CREATE FUNCTION`](create-function.html) statements, but has no effect in such cases.

   Prior to MySQL 8.0.28, it was possible to create a stored function with the same name as an existing loadable function, but not the other way around (Bug #33301931 ).

 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 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`](drop-function.html) to drop the loadable function and [`CREATE FUNCTION`](create-function.html) to re-create the loadable function 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 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.

