Beginning SQL

8
Queries within Queries
This chapter examines how you can nest one query inside another. SQL allows queries within
queries, or subqueries, which are SELECT statements inside SELECT statements. This might sound
a bit odd, but subqueries can actually be very useful. The downside, however, is that they can
consume a lot of processing, disk, and memory resources.
A subquery’s syntax is just the same as a normal SELECT query’s syntax. As with a normal SELECT
statement, a subquery can contain joins, WHERE clauses, HAVING clauses, and GROUP BY clauses.
Specifically, this chapter shows you how to use subqueries with SELECT statements, either returning
results inside a column list or helping filter results when used inside a WHERE or HAVING
clause; to update and delete data by using subqueries with UPDATE and DELETE statements; and
with operators such as EXISTS, ANY, SOME, and ALL, which are introduced later in this chapter.
Subqueries are particularly powerful when coupled with SQL operators such as IN, ANY, SOME,
and ALL, which are covered shortly. However, the chapter begins with a few easy examples of
subqueries. Before starting, you should note that versions of MySQL prior to version 4.1 do not
fully support subqueries and many of the examples in this chapter won’t work on early versions
of MySQL.
Subquery Terminology
Throughout this chapter, you’ll notice references to the outer and inner subqueries. The outer
query is the main SELECT statement, and you could say that so far all of your SELECT statements
have been outer queries. Shown below is a standard query:
SELECT MemberId FROM Members;
Using the standard query, you can nest — that is, place inside the outer query — a subquery, which
is termed the inner query:
SELECT MemberId FROM MemberDetails
WHERE MemberId = (SELECT MAX(FilmId) FROM Films);