What is a subquery?

What is a subquery?

• A. A SELECT statement that uses only constants.
• B. A query that joins two tables directly without a subquery.
• C. A SELECT statement embedded inside another SELECT statement.
• D. A query that references only a single table and no subquery.

Answer: (C)

A subquery is a SELECT statement embedded inside another SELECT statement. It lets the inner query run first to produce data that the outer query uses, or to compute a value used by the outer query. For example, you can filter rows based on values produced by the inner query: select name from employees where department_id in (select id from departments where location = 'New York'). Subqueries can appear in different parts of a statement, such as the WHERE clause, the FROM clause (where the inner query acts as a derived table), or even the SELECT clause when you need a computed value. They can return a single value (scalar) or multiple rows, and they can be correlated (referencing outer query columns) or uncorrelated (independent). The key point is that the data or value comes from a query nested inside another query. The other descriptions describe queries that operate on constants, join directly without nesting, or reference only a single table, which do not involve nesting a SELECT inside another query.

A subquery is a SELECT statement embedded inside another SELECT statement. It lets the inner query run first to produce data that the outer query uses, or to compute a value used by the outer query. For example, you can filter rows based on values produced by the inner query: select name from employees where department_id in (select id from departments where location = 'New York'). Subqueries can appear in different parts of a statement, such as the WHERE clause, the FROM clause (where the inner query acts as a derived table), or even the SELECT clause when you need a computed value. They can return a single value (scalar) or multiple rows, and they can be correlated (referencing outer query columns) or uncorrelated (independent). The key point is that the data or value comes from a query nested inside another query. The other descriptions describe queries that operate on constants, join directly without nesting, or reference only a single table, which do not involve nesting a SELECT inside another query.