23.3.2.2. UDF Calling Sequences for Aggregate Functions

This section describes the different functions that you need to define when you create an aggregate UDF. Section 23.3.2, "Adding a New User-Defined Function", describes the order in which MySQL calls these functions.

xxx_reset()

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:
```
void xxx_reset(UDF_INIT *initid, UDF_ARGS *args,               char *is_null, char *error);
```
xxx_reset() is not needed or used in MySQL 5.6, in which the UDF interface uses xxx_clear() instead. However, you can define both xxx_reset() and xxx_clear() if you want to have your UDF work with older versions of the server. (If you do include both functions, the xxx_reset() function in many cases can be implemented internally by calling xxx_clear() to reset all variables, and then calling xxx_add() to add the UDF_ARGS argument as the first value in the group.)
xxx_clear()

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:
```
void xxx_clear(UDF_INIT *initid, char *is_null, char *error);
```
is_null is set to point to CHAR(0) before calling xxx_clear().

If something went wrong, you can store a value in the variable to which the error argument points. error points to a single-byte variable, not to a string buffer.

xxx_clear() is required by MySQL 5.6.
xxx_add()

This function is called for all rows that belong to the same group. You should use it to add the value in the UDF_ARGS argument to your internal summary variable.
```
void xxx_add(UDF_INIT *initid, UDF_ARGS *args,char *is_null, char *error);
```

The xxx() function for an aggregate UDF should be declared the same way as for a nonaggregate UDF. See Section 23.3.2.1, "UDF Calling Sequences for Simple Functions".

For an aggregate UDF, MySQL calls the xxx() function after all rows in the group have been processed. You should normally never access its UDF_ARGS 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 nonaggregate UDF. See Section 23.3.2.4, "UDF Return Values and Error Handling".

The xxx_reset() and xxx_add() functions handle their UDF_ARGS argument the same way as functions for nonaggregate UDFs. See Section 23.3.2.3, "UDF Argument Processing".

The pointer arguments to is_null and error are the same for all calls to xxx_reset(), xxx_clear(), xxx_add() and xxx(). You can use this to remember that you got an error or whether the xxx() function should return NULL. You should not store a string into *error! error points to a single-byte variable, not to a string buffer.

*is_null is reset for each group (before calling xxx_clear()). *error is never reset.

If *is_null or *error are set when xxx() returns, MySQL returns NULL as the result for the group function.