SQL-AG Approach

SQL-AG allows the user to define new aggregates in SQL. It is an extension of SQL supporting extensible, user-definable aggregates. The system was developed using Oracle DBMS and PL/SQL as the implementation environment. All features of SQL-AG are described in Chapter 3.

In this approach the user first defines the aggregate using a specification language SQL-AG, described in Section 3.3. Then using SQL-AG the user generates an aggregate in SQL and can use the aggregate in an equivalent way as a built-in aggregate. This approach is similar to what SQL3 has proposed, but several extensions have been introduce to make it more powerful using the lessons learnt from LDL++. SQL-AG approach also uses an iterative routine. The iterative routine is composed of three routines namely single, multi and produce. As described in more detail in later chapters, these three routines are similar to initialize, iterate and terminate routines of SQL3. The following are the key differences in the two approaches:

• unlike initialize, single computes an initial value based on the first record,
• unlike terminate, produce returns partial results depending on intermediate states, rather than just a final result depending on the final state.