Task 6 - Subqueries and Nested Queries

Objective

Practice SQL subqueries in different clauses (SELECT, WHERE, FROM) and understand scalar, correlated, and derived tables.

1.What is a subquery?

A subquery is a query inside another SQL query which is used to fetch results that the main query depends on.

2.Difference between subquery and join?

A sub query is nested and runs separately and is mainly used for filtering data and join is used to combine rows from multiple tables in one result set.

3.What is a correlated subquery?

When a subquery depends upon the outer query for its values and runs per row of the outer query.

4.Can subqueries return multiple rows?

Yes subqueries can result in multiple rows.

5.How does EXISTS work?

It checks if the subquery returns at least one row.

6.How is performance affected by subqueries?

Subqueries are slower because they may run multiple times.

7.What is scalar subquery?

A scalar subquery returns a single value.

8.Where can we use subqueries?

They are used in select, where, from and having clauses.

9.Can a subquery be in FROM clause?

Yes, it is known as derived table.

10.What is a derived table?

A derived table is a temporary table created using a subquery in the from clause used like a normal table.

Screenshots

1. Subquery

2. Corelated subquery

3. Exists

4. Scalar subquery

5. Subquery in from clause

6. Derived table