Adding an aggregate function requires some additional effort as an aggregate function on the SQL level is actually implemented by a small family of C functions and private data passed between them. Aggregate private data is declared using the <data> tag. For each aggregate group it is initialized by the code given in <clear>. For each row of the group the <add> code is executed, receiving the declared <parameter>s just like a normal function. Finally the <result> code is executed and expected to pass the functions. See the following example for a simple reimplementation of the standard AVG() function: