ASA SQL User's Guide
Using Subqueries
Understanding which queries are valid and which ones aren't can be complicated when a query contains a subquery. Similarly, figuring out what a multi-level query does can also be very involved, and it helps to understand how Adaptive Server Anywhere processes subqueries. For general information about processing queries, see Summarizing, Grouping and Sorting Query Results.
Correlated subqueries
Converting subqueries in the WHERE clause to joins
Comparison operators
Quantified comparison test
Set membership test
Existence test