A subquery is a
SELECT statement embedded within another
SELECT statement. The embedded subquery is often referenced as the query's inner statement, while the containing query is typically referenced as the query's statement, or outer query block. A subquery returns data that the outer query uses as a condition to determine what data to retrieve. There is no limit to the number of nested subqueries you can create.
Like any query, a subquery returns records from a table that might consist of a single column and record, a single column with multiple records, or multiple columns and records. Queries can be noncorrelated or correlated. You can even use them to update or delete records in a table based on values that are stored in other database tables.
Many examples in this section use the VMart database.