subqueries_flashcards

Question 1

What is a subquery?

Answer 1

A SELECT query nested inside another query.

Question 2

Where can subqueries be used?

Answer 2

In SELECT, FROM, and WHERE clauses.

Question 3

What is a scalar subquery?

Answer 3

A subquery that returns a single value.

Question 4

What is a correlated subquery?

Answer 4

A subquery that refers to columns from the outer query.

Question 5

When should you use IN in a subquery?

Answer 5

When comparing a column to multiple values returned by a subquery.

Question 6

What is the difference between = and IN in subqueries?

Answer 6

= is used for scalar (one result), IN is used for multiple-row subqueries.

Question 7

Can you use ORDER BY inside a subquery?

Answer 7

No, unless it’s in a subquery used in the FROM clause.

Question 8

How do subqueries and joins differ?

Answer 8

Subqueries are nested; joins combine tables horizontally.

Question 9

Give an example of a correlated subquery.

Answer 9

SELECT name FROM employee e WHERE salary > (SELECT AVG(salary) FROM employee WHERE dept = e.dept);