Subqueries and nested queries in DBMS Theory - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a subquery in SQL?

A subquery is a query written inside another query. It helps to get data that will be used by the main query.

beginner

How does a nested query differ from a simple query?

A nested query is a query inside another query, while a simple query stands alone without any inner queries.

intermediate

Can subqueries return multiple rows or columns?

Yes, subqueries can return one or many rows and columns depending on how they are written and used.

intermediate

Where can subqueries be used in SQL statements?

Subqueries can be used in SELECT, WHERE, FROM, and HAVING clauses to filter, calculate, or join data.

advanced

What is the difference between correlated and non-correlated subqueries?

A correlated subquery depends on the outer query for its values and runs once per row, while a non-correlated subquery runs independently once.

What is a subquery in SQL?

A A query that updates data

B A query inside another query

C A query that deletes tables

D A query that creates indexes

A subquery is a query written inside another query to help filter or calculate data.

Where can subqueries be used in SQL?

A Only in SELECT clause

B Only in WHERE clause

C In SELECT, WHERE, FROM, and HAVING clauses

D Only in CREATE statements

Subqueries can be used in multiple clauses like SELECT, WHERE, FROM, and HAVING.

What does a correlated subquery depend on?

A It depends on the user input

B It runs independently

C It depends on the database engine

D It depends on the outer query

A correlated subquery uses values from the outer query and runs once per row.

Can subqueries return multiple rows?

A Yes, they can return multiple rows

B They return no rows

C Only one column

D No, only one row

Subqueries can return one or many rows depending on their design.

What is a nested query?

A A query inside another query

B A query that deletes data

C A query that creates tables

D A query that runs outside the database

A nested query is simply a query inside another query.

Explain what a subquery is and give an example of where it might be used.

Think about how one query can help another by providing data.

Describe the difference between correlated and non-correlated subqueries.

Consider if the subquery depends on the outer query or not.