This section describes the different functions that you need to define when you create an aggregate UDF. 「Adding a New User-Defined Function」, describes the order in which MySQL calls these functions.
This function is called when MySQL finds the first row in
a new group. It should reset any internal summary
variables and then use the given
UDF_ARGS argument as the first value in
your internal summary value for the group. Declare
xxx_reset() as follows:
char *xxx_reset(UDF_INIT *initid, UDF_ARGS *args, char *is_null, char *error);
xxx_reset() is not needed or used in
MySQL 5.1, in which the UDF interface uses
xxx_clear() instead. However, you can
xxx_clear() if you want to have your
UDF work with older versions of the server. (If you do
include both functions, the
function in many cases can be implemented internally by
xxx_clear() to reset all
variables, and then calling
to add the
UDF_ARGS argument as the
first value in the group.)
This function is called when MySQL needs to reset the
summary results. It is called at the beginning for each
new group but can also be called to reset the values for a
query where there were no matching rows. Declare
xxx_clear() as follows:
char *xxx_clear(UDF_INIT *initid, char *is_null, char *error);
is_null is set to point to
CHAR(0) before calling
If something went wrong, you can store a value in the
variable to which the
error points to a single-byte
variable, not to a string buffer.
xxx_clear() is required by MySQL
This function is called for all rows that belong to the
same group, except for the first row. You should use it to
add the value in the
to your internal summary variable.
char *xxx_add(UDF_INIT *initid, UDF_ARGS *args, char *is_null, char *error);
xxx() function for an aggregate UDF
should be declared the same way as for a non-aggregate UDF.
See 「UDF Calling Sequences for Simple Functions」.
For an aggregate UDF, MySQL calls the
function after all rows in the group have been processed. You
should normally never access its
argument here but instead return a value based on your
internal summary variables.
Return value handling in
xxx() should be
done the same way as for a non-aggregate UDF. See
「UDF Return Values and Error Handling」.
xxx_add() functions handle their
UDF_ARGS argument the same way as functions
for non-aggregate UDFs. See 「UDF Argument Processing」.
The pointer arguments to
error are the same for all calls to
xxx(). You can use this to remember
that you got an error or whether the
function should return
NULL. You should not
store a string into
error points to a single-byte variable, not
to a string buffer.
*is_null is reset for each group (before
*error is never reset.
are set when
xxx() returns, MySQL returns
NULL as the result for the group function.