9.2.3 Function Name Parsing and Resolution

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 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:

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 or stored 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:

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 5.1.7, “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:

Function Name Resolution

The following rules describe how the server resolves references to function names for function creation and invocation:

The preceding function name resolution rules have implications for upgrading to versions of MySQL that implement new built-in functions: