Using Subqueries as Column Expressions
In Chapters 4, 5, and 6, you learned that the items in a SELECT-clause list can be literals, column names, or expressions. SQL also permits you to embed a subquery in a SELECT-clause list.
A subquery that’s used as a column expression must be a scalar subquery. Recall from Table 8.1 in “Subquery Syntax” earlier in this chapter that a scalar subquery returns a single value (that is, a one-row, one-column result). In most cases, you’ll have to use an aggregate function or restrictive WHERE conditions in the subquery to guarantee that the subquery returns only one row.
The syntax for the SELECT-clause list is the same as you’ve been using all along, except that you may specify a parenthesized subquery as one ...
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