Subquery in FROM clause (derived table) in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a subquery in the FROM clause, also known as a derived table?

A subquery in the FROM clause is a query nested inside the FROM part of a main query. It acts like a temporary table that the main query can use to get results.

beginner

Why do we use a subquery in the FROM clause?

We use it to simplify complex queries by breaking them into smaller parts. It helps to organize data before applying filters or joins in the main query.

beginner

How do you give a name to a derived table in SQL?

You give a derived table an alias right after the closing parenthesis of the subquery. This alias is used to refer to the derived table in the main query.

intermediate

Example: What does this SQL do? SELECT dt.category, dt.total_sales FROM (SELECT category, SUM(sales) AS total_sales FROM products GROUP BY category) AS dt WHERE dt.total_sales > 1000;

This query first calculates total sales per category inside the subquery (derived table). Then, the main query selects categories where total sales are more than 1000.

intermediate

Can a derived table be used with JOINs in the main query?

Yes, a derived table can be joined with other tables or derived tables in the main query to combine and filter data effectively.

What is the purpose of a subquery in the FROM clause?

A To update data in the database

B To create a temporary table for the main query to use

C To filter rows after the main query runs

D To delete rows from a table

A subquery in the FROM clause creates a temporary table (derived table) that the main query can use.

How do you refer to a derived table in the main query?

A By the column names

B By its original table name

C By the database name

D By an alias given after the subquery

You must give the derived table an alias and use that alias to refer to it in the main query.

Which SQL keyword is used to name a derived table?

A AS

B FROM

C WHERE

D GROUP BY

The AS keyword is used to assign an alias to the derived table.

Can a derived table contain aggregate functions like SUM or COUNT?

A Yes, it can perform aggregations

B No, aggregates are not allowed

C Only COUNT is allowed

D Only SUM is allowed

Derived tables can include aggregate functions to summarize data before the main query uses it.

Is this SQL valid? SELECT * FROM (SELECT id FROM users) WHERE id > 10;

A Yes, it is valid

B No, subqueries cannot be in FROM

C No, the derived table needs an alias

D No, WHERE cannot be used here

Derived tables must have an alias to be valid in SQL.

Explain what a derived table is and how it is used in SQL queries.

Think about how you can create a small table inside a query to help with filtering or joining.

Describe the steps to write a query using a subquery in the FROM clause with an example.

Start with a simple aggregation inside the subquery, then filter results in the main query.

Practice

1. What is the main purpose of using a subquery in the FROM clause in SQL?

easy

A. To update values in a table

B. To permanently store data in the database

C. To delete rows from a table

D. To create a temporary table that can be used by the main query

Solution

1. Step 1: Understand the role of subqueries in FROM clause

   Subqueries in the FROM clause act like temporary tables that the main query can use to simplify complex operations.

2. Step 2: Differentiate from other SQL operations

   Unlike DELETE or UPDATE, subqueries in FROM do not modify data but help organize data for selection.

3. Final Answer:

   To create a temporary table that can be used by the main query -> Option D

4. Quick Check:

   Subquery in FROM = temporary table [OK]

Hint: Subquery in FROM creates a temp table for main query use [OK]

Common Mistakes:

• Thinking subquery stores data permanently
• Confusing subquery with DELETE or UPDATE commands
• Forgetting subquery is temporary, not permanent

2. Which of the following is the correct syntax for using a subquery in the FROM clause?

easy

A. SELECT * FROM (SELECT id FROM users) AS sub;

B. SELECT * FROM users WHERE (SELECT id FROM users);

C. SELECT * FROM users AS (SELECT id FROM users);

D. SELECT * FROM users (SELECT id FROM users);

Solution

1. Step 1: Identify correct subquery syntax in FROM

   The subquery must be enclosed in parentheses and given an alias using AS.

2. Step 2: Check each option

   SELECT * FROM (SELECT id FROM users) AS sub; correctly uses parentheses and alias. Others misuse WHERE, alias placement, or parentheses.

3. Final Answer:

   SELECT * FROM (SELECT id FROM users) AS sub; -> Option A

4. Quick Check:

   Subquery in FROM needs parentheses + alias [OK]

Hint: Subquery in FROM needs parentheses and alias [OK]

Common Mistakes:

• Omitting alias for subquery
• Placing subquery in WHERE instead of FROM
• Incorrect alias placement

3. Given the tables:
users(id, name)
orders(id, user_id, amount)
What will this query return?

SELECT sub.name, sub.total FROM (SELECT u.name, SUM(o.amount) AS total FROM users u JOIN orders o ON u.id = o.user_id GROUP BY u.name) AS sub WHERE sub.total > 100;

medium

A. Names of users with total order amount greater than 100

B. All users with their total order amount

C. Syntax error due to missing alias

D. Empty result because no users have orders

Solution

1. Step 1: Understand the subquery

   The subquery calculates total order amount per user by joining users and orders and grouping by user name.

2. Step 2: Apply the outer WHERE filter

   The outer query filters to only include users whose total order amount is greater than 100.

3. Final Answer:

   Names of users with total order amount greater than 100 -> Option A

4. Quick Check:

   Subquery sums orders; outer filters total > 100 [OK]

Hint: Subquery calculates totals; outer query filters results [OK]

Common Mistakes:

• Ignoring the WHERE filter on total
• Assuming all users are returned
• Confusing alias usage

4. Identify the error in this query:

SELECT sub.name, sub.total FROM (SELECT name, SUM(amount) AS total FROM users JOIN orders ON users.id = orders.user_id) sub;

medium

A. Missing alias for subquery

B. Incorrect JOIN syntax

C. Missing GROUP BY clause in subquery

D. Using subquery in WHERE clause instead of FROM

Solution

1. Step 1: Analyze the subquery aggregation

   The subquery uses SUM(amount) but does not group by name, which is required when selecting non-aggregated columns.

2. Step 2: Confirm alias and JOIN syntax

   The subquery has an alias 'sub' and JOIN syntax is correct, so these are not errors.

3. Final Answer:

   Missing GROUP BY clause in subquery -> Option C

4. Quick Check:

   Aggregation needs GROUP BY for non-aggregated columns [OK]

Hint: Aggregation needs GROUP BY for other selected columns [OK]

Common Mistakes:

• Forgetting GROUP BY with aggregation
• Confusing alias requirement
• Misreading JOIN syntax

5. You want to find the average order amount per user but only for users who have placed more than 3 orders. Which query correctly uses a subquery in the FROM clause to achieve this?

hard

A. SELECT user_id, AVG(amount) FROM (SELECT * FROM orders WHERE COUNT(*) > 3) AS sub GROUP BY user_id;

B. SELECT sub.user_id, sub.avg_amount FROM (SELECT user_id, AVG(amount) AS avg_amount FROM orders GROUP BY user_id HAVING COUNT(*) > 3) AS sub;

C. SELECT user_id, AVG(amount) FROM orders WHERE COUNT(*) > 3 GROUP BY user_id;

D. SELECT user_id, AVG(amount) FROM orders GROUP BY user_id HAVING AVG(amount) > 3;

Solution

1. Step 1: Understand the requirement

   We need average order amount per user but only for users with more than 3 orders.

2. Step 2: Analyze each option

   SELECT sub.user_id, sub.avg_amount FROM (SELECT user_id, AVG(amount) AS avg_amount FROM orders GROUP BY user_id HAVING COUNT(*) > 3) AS sub; correctly uses a subquery to group orders by user_id, filters users with more than 3 orders using HAVING, then calculates average amount.

3. Final Answer:

   SELECT sub.user_id, sub.avg_amount FROM (SELECT user_id, AVG(amount) AS avg_amount FROM orders GROUP BY user_id HAVING COUNT(*) > 3) AS sub; -> Option B

4. Quick Check:

   Subquery filters users by order count, outer selects average [OK]

Hint: Use HAVING in subquery to filter groups before outer select [OK]

Common Mistakes:

• Using WHERE with aggregation functions
• Placing HAVING outside GROUP BY context
• Not using subquery to filter groups first