A subquery is a query nested within another query. The containing query is called an outer query. A subquery in turn can have a nested query, making it a multiply nested query.
This chapter discusses subqueries in detail.
A single-row subquery is a subquery that returns a single value. A single-row subquery can be placed in the WHERE clause of an outer query. The return value of the subquery is compared with a column of the outer query using one of the comparison operators. (Comparison operators were discussed in Chapter 2, �Basic Queries�)
For example, the query in Listing 6.1 contains a single-row subquery that returns the highest sale price recorded for a product. The outer query returns all products ...