Common Table Expressions (WITH) in MySQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a Common Table Expression (CTE) in SQL?

A Common Table Expression (CTE) is a temporary named result set that you can reference within a SELECT, INSERT, UPDATE, or DELETE statement. It helps organize complex queries by breaking them into simpler parts.

beginner

How do you start a CTE in SQL?

You start a CTE with the keyword WITH, followed by the CTE name, optional column names, and the query that defines it inside parentheses.

intermediate

Can a CTE refer to itself in MySQL?

Yes, in MySQL 8.0 and later, CTEs can be recursive, meaning they can refer to themselves to perform repeated operations like traversing hierarchical data.

intermediate

Why use a CTE instead of a subquery?

CTEs improve readability by naming parts of a query, making complex queries easier to understand and maintain. They can also be referenced multiple times within the main query.

beginner

Write a simple example of a CTE that selects all employees with salary over 50000.

WITH HighEarners AS (SELECT * FROM employees WHERE salary > 50000) SELECT * FROM HighEarners;

What keyword starts a Common Table Expression in SQL?

A WITH

B SELECT

C FROM

D JOIN

The keyword WITH is used to start a CTE.

Which of the following is a benefit of using CTEs?

A They speed up database backups.

B They permanently store data.

C They replace indexes.

D They improve query readability.

CTEs help make complex queries easier to read and maintain.

Can a CTE be recursive in MySQL 8.0+?

A Only if you use a stored procedure.

B Yes, it can refer to itself.

C Only in MySQL 5.x versions.

D No, recursion is not allowed.

MySQL 8.0 and later support recursive CTEs.

How do you reference a CTE inside the main query?

A By its name defined after WITH.

B By using the keyword SUBQUERY.

C By using the keyword TEMP.

D By using the table name only.

You use the CTE name defined after WITH to reference it.

Which statement is true about CTEs?

A CTEs permanently store data in the database.

B CTEs replace the need for indexes.

C CTEs can be used multiple times in the same query.

D CTEs are only used for deleting data.

CTEs are temporary and can be referenced multiple times within the query.

Explain what a Common Table Expression (CTE) is and why it is useful in SQL queries.

Think about how breaking a big task into smaller parts helps.

Describe the syntax of a CTE and how you use it in a SELECT statement.

Start with WITH, then name, then the query, then use the name in your main query.

Practice

1. What is the main purpose of using a WITH clause (Common Table Expression) in a MySQL query?

easy

A. To permanently store data in a new table

B. To create a temporary named result set that can be referenced within the main query

C. To delete rows from a table

D. To create an index on a table column

Solution

1. Step 1: Understand the role of WITH clause

   The WITH clause defines a temporary named result set, called a Common Table Expression (CTE), which exists only during the execution of the query.

2. Step 2: Differentiate from permanent operations

   Unlike creating tables or indexes, CTEs do not store data permanently but help organize complex queries by breaking them into parts.

3. Final Answer:

   To create a temporary named result set that can be referenced within the main query -> Option B

4. Quick Check:

   WITH clause = temporary named result set [OK]

Hint: WITH creates temporary named results for easier queries [OK]

Common Mistakes:

• Thinking WITH creates permanent tables
• Confusing WITH with DELETE or UPDATE commands
• Assuming WITH creates indexes

2. Which of the following is the correct syntax to start a Common Table Expression in MySQL?

easy

A. DEFINE cte_name AS SELECT * FROM table_name

B. CREATE CTE cte_name SELECT * FROM table_name

C. START CTE cte_name (SELECT * FROM table_name)

D. WITH cte_name AS (SELECT * FROM table_name)

Solution

1. Step 1: Recall the standard syntax for CTE

   The correct syntax to define a CTE starts with the keyword WITH, followed by the CTE name, AS, and then the query in parentheses.

2. Step 2: Compare options with syntax

   Only WITH cte_name AS (SELECT * FROM table_name) matches the correct syntax: WITH cte_name AS (SELECT * FROM table_name). Other options use invalid keywords or structure.

3. Final Answer:

   WITH cte_name AS (SELECT * FROM table_name) -> Option D

4. Quick Check:

   WITH + name + AS + (query) = correct syntax [OK]

Hint: WITH cte_name AS (query) is the standard start [OK]

Common Mistakes:

• Using CREATE or DEFINE instead of WITH
• Omitting AS keyword
• Not enclosing query in parentheses

3. Given the following query, what will be the output?

WITH SalesCTE AS (
  SELECT product_id, SUM(quantity) AS total_qty
  FROM sales
  GROUP BY product_id
)
SELECT * FROM SalesCTE WHERE total_qty > 100;

medium

A. All products with total quantity sold greater than 100

B. All products with total quantity sold less than or equal to 100

C. Syntax error due to missing GROUP BY

D. Empty result because CTE is not used properly

Solution

1. Step 1: Analyze the CTE definition

   The CTE named SalesCTE calculates total quantity sold per product by grouping sales by product_id and summing quantity.

2. Step 2: Understand the main query filter

   The main query selects rows from SalesCTE where total_qty is greater than 100, so only products with sales above 100 units appear.

3. Final Answer:

   All products with total quantity sold greater than 100 -> Option A

4. Quick Check:

   CTE sums and filters total_qty > 100 [OK]

Hint: CTE calculates sums; main query filters by condition [OK]

Common Mistakes:

• Confusing > 100 with <= 100
• Thinking GROUP BY is missing (it is present)
• Assuming CTE is unused or causes error

4. Identify the error in the following MySQL query using a CTE:

WITH cte AS (
  SELECT id, name FROM users
)
SELECT * FROM cte WHERE age > 30;

medium

A. CTE query missing a semicolon after closing parenthesis

B. WITH clause must be followed by INSERT statement

C. Column 'age' does not exist in the CTE result

D. CTE name cannot be 'cte'

Solution

1. Step 1: Check columns selected in CTE

   The CTE selects only 'id' and 'name' columns from users table; 'age' is not included.

2. Step 2: Analyze main query filter

   The main query tries to filter on 'age', which is not part of the CTE result, causing an error.

3. Final Answer:

   Column 'age' does not exist in the CTE result -> Option C

4. Quick Check:

   Filter column must be in CTE output [OK]

Hint: Filter columns must be selected in CTE first [OK]

Common Mistakes:

• Assuming semicolon needed inside CTE
• Thinking CTE name 'cte' is invalid
• Believing WITH must precede INSERT

5. You want to find the top 3 customers by total purchase amount using a CTE. Which query correctly uses a CTE to achieve this?

hard

A. WITH CustomerTotals AS (
SELECT customer_id, SUM(amount) AS total_amount
FROM orders
GROUP BY customer_id
)
SELECT customer_id, total_amount
FROM CustomerTotals
ORDER BY total_amount DESC
LIMIT 3;

B. WITH CustomerTotals AS (
SELECT customer_id, amount
FROM orders
)
SELECT customer_id, SUM(amount) AS total_amount
FROM CustomerTotals
GROUP BY customer_id
ORDER BY total_amount DESC
LIMIT 3;

C. WITH CustomerTotals AS (
SELECT customer_id, SUM(amount) AS total_amount
FROM orders
)
SELECT * FROM CustomerTotals
LIMIT 3;

D. WITH CustomerTotals AS (
SELECT customer_id, SUM(amount) AS total_amount
FROM orders
GROUP BY customer_id
ORDER BY total_amount DESC
LIMIT 3
)
SELECT * FROM CustomerTotals;

Solution

1. Step 1: Check CTE calculation and grouping

   WITH CustomerTotals AS ( SELECT customer_id, SUM(amount) AS total_amount FROM orders GROUP BY customer_id ) SELECT customer_id, total_amount FROM CustomerTotals ORDER BY total_amount DESC LIMIT 3; correctly groups orders by customer_id and sums amount inside the CTE.

2. Step 2: Verify ordering and limiting in main query

   WITH CustomerTotals AS ( SELECT customer_id, SUM(amount) AS total_amount FROM orders GROUP BY customer_id ) SELECT customer_id, total_amount FROM CustomerTotals ORDER BY total_amount DESC LIMIT 3; orders by total_amount descending and limits to top 3 customers outside the CTE, which is valid in MySQL.

3. Step 3: Analyze other options for errors

   WITH CustomerTotals AS ( SELECT customer_id, amount FROM orders ) SELECT customer_id, SUM(amount) AS total_amount FROM CustomerTotals GROUP BY customer_id ORDER BY total_amount DESC LIMIT 3; does not sum inside CTE, causing incorrect grouping. WITH CustomerTotals AS ( SELECT customer_id, SUM(amount) AS total_amount FROM orders ) SELECT * FROM CustomerTotals LIMIT 3; misses GROUP BY and ordering. WITH CustomerTotals AS ( SELECT customer_id, SUM(amount) AS total_amount FROM orders GROUP BY customer_id ORDER BY total_amount DESC LIMIT 3 ) SELECT * FROM CustomerTotals; tries to use ORDER BY and LIMIT inside CTE, which is invalid in MySQL.

4. Final Answer:

   Option A query correctly finds top 3 customers by total purchase amount -> Option A

5. Quick Check:

   Group and sum in CTE, order and limit outside [OK]

Hint: Group and sum in CTE; order and limit outside [OK]

Common Mistakes:

• Placing ORDER BY and LIMIT inside CTE
• Not grouping before summing
• Trying to sum outside CTE without grouping