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
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.