Welcome to our free Advanced Oracle SQL Queries tutorial. This tutorial is based on Webucator's Advanced Oracle SQL Queries course.
A subquery is a SQL SELECT statement that is contained within another SELECT statement. Subqueries can appear in various parts of a query, including the SELECT clause, the FROM clause, the WHERE clause and the HAVING clause. The innermost query is evaluated prior to queries that contain it. Performance problems can result if an "expensive" query is nested as an inner query.