Sub Query

Question 1

What is the primary purpose of a **subquery** in SQL?

A.

A) To replace the JOIN operation entirely

B.

B) To nest a query inside another query, with the inner query executing first to provide data

**Explanation:** A subquery is a query nested inside another query. The inner query executes first and returns a result (scalar value, list, or table) that the outer query uses. Subqueries simplify complex logic by breaking problems into manageable parts.

C.

C) To create a temporary table that persists in the database

D.

D) To speed up query execution by eliminating WHERE clauses

Question 2

What does a **scalar subquery** return when used in a WHERE clause?

A.

A) Multiple rows and columns

B.

B) A list of values for use with IN or NOT IN

C.

C) A single value that can be compared with operators like >, <, or =

Explanation: A scalar subquery returns exactly one value (one row, one column), making it suitable for direct comparison with operators. For example, `WHERE Salary > (SELECT MAX(Salary) FROM Employees)` compares a single maximum salary value.

D.

D) An error because scalar subqueries cannot be used in WHERE

Question 3

What is the primary purpose of using a subquery in the **HAVING** clause?

A.

A) To compare aggregated values of groups against a scalar value

Explanation: The **HAVING** clause filters groups after aggregation, and a subquery provides a single scalar value for comparison, such as average salary across departments.

B.

B) To filter individual rows before grouping

C.

C) To join multiple tables within the group

D.

D) To sort the grouped results

Question 4

What is the primary purpose of using a subquery in the **select clause**?

A.

A. To filter rows in the outer query before execution

B.

B. To compute a single value per row of the outer query using correlated references

Explanation: Subqueries in the select clause embed a nested query that returns one scalar value for each outer row, linked via aliases in the WHERE condition.

C.

C. To join multiple tables without aliases

D.

D. To group results across the entire dataset

Question 5

What is the primary requirement for a subquery placed in the **FROM clause**?

A.

A) It must return a single scalar value

B.

B) It must return a complete table structure with rows and columns

**Explanation:** Subqueries in FROM act as derived tables, requiring a full table structure rather than scalars or lists to enable joining.

C.

C) It must contain only aggregate functions

D.

D) It must reference the outer query's tables

📝 Sub Query

Quiz Complete!