Which statement describes a scalar subquery in the SELECT clause?

• A. A subquery that returns a single value to be used in the row.
• B. A subquery that returns multiple values to be used in the SELECT.
• C. A subquery that replaces the FROM clause.
• D. A subquery that is evaluated after the outer query completes.

Answer: (A)

A scalar subquery in the SELECT clause is a subquery that returns exactly one value, and that single value becomes the value shown in the result for each row of the outer query. It’s typically correlated with the outer query, so it can compute a value based on that row, but it can also be uncorrelated and return a constant. Because it must return one value, the inner query cannot produce more than one row; otherwise you’d get an error.

This is why describing it as a subquery that returns a single value to be used in the row is the best fit. It isn’t about replacing the FROM clause—that would be a derived table or inline view—and it isn’t about evaluating after the outer query completes; the subquery in the SELECT list is evaluated as part of producing each row (often per row if correlated). For example, you might compute a department’s average salary for each employee with a subquery in the SELECT list, yielding one value per row.