MySQL 4.1 supports built-in (native) functions and user-defined functions (UDFs). 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 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:
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 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 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 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
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 5.1.7, “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:
Avoid creating UDFs 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 server resolves references to function names for function
creation and invocation as follows: 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.
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 nonconflicting name.
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