SQL All-In-One For Dummies, 3rd Edition

Chapter 3

Querying Multiple Tables with Subqueries

IN THIS CHAPTER

Defining subqueries

Discovering how subqueries work

Nesting subqueries

Tuning nested subqueries

Tuning correlation subqueries

Relational databases have multiple tables. That’s where the word relational comes from — multiple tables that relate to each other in some way. One consequence of the distribution of data across multiple tables is that most queries need to pull data from more than one of them. There are a couple of ways to do this. One is to use relational operators, which I cover in the next chapter. The other method is to use subqueries, which is the subject of this chapter.

What Is a Subquery?

A subquery is an SQL statement embedded within another SQL statement. It’s possible for a subquery to be embedded within another subquery, which is in turn embedded within an outermost SQL statement. Theoretically, there is no limit to the number of levels of subquery that an SQL statement may include, although any ...