Common Table Expressions (WITH) in MySQL

Introduction

Common Table Expressions (CTEs) help you organize complex queries by creating temporary named result sets you can use inside your main query.

When you want to break a big query into smaller, easier parts.

When you need to reuse the same subquery multiple times in one query.

When you want to improve the readability of your SQL code.

When you want to write recursive queries, like finding all parts in a product hierarchy.

When you want to avoid repeating the same code in multiple places.

Syntax

MySQL

WITH cte_name AS (
  SELECT column1, column2
  FROM table_name
  WHERE condition
)
SELECT * FROM cte_name;

The CTE is defined after the WITH keyword and before the main SELECT.

You can use the CTE name like a temporary table in the main query.

Examples

This example creates a CTE named recent_orders to get orders after January 1, 2024, then selects all from it.

MySQL

WITH recent_orders AS (
  SELECT order_id, customer_id, order_date
  FROM orders
  WHERE order_date > '2024-01-01'
)
SELECT * FROM recent_orders;

This example finds customers who spent more than 1000 using a CTE, then selects their IDs.

MySQL

WITH top_customers AS (
  SELECT customer_id, SUM(amount) AS total_spent
  FROM sales
  GROUP BY customer_id
  HAVING SUM(amount) > 1000
)
SELECT customer_id FROM top_customers;

This example defines two CTEs for different departments and combines their results.

MySQL

WITH cte1 AS (
  SELECT * FROM employees WHERE department = 'Sales'
),
cte2 AS (
  SELECT * FROM employees WHERE department = 'HR'
)
SELECT * FROM cte1
UNION ALL
SELECT * FROM cte2;

Sample Program

This query uses a CTE named high_salary to find employees earning more than 70000, then lists them ordered by salary.

MySQL

WITH high_salary AS (
  SELECT employee_id, salary
  FROM employees
  WHERE salary > 70000
)
SELECT employee_id, salary FROM high_salary ORDER BY salary DESC;

Important Notes

CTEs exist only during the execution of the query; they do not create permanent tables.

You can define multiple CTEs separated by commas after WITH.

MySQL supports CTEs starting from version 8.0.

Summary

CTEs help organize and simplify complex SQL queries.

They create temporary named result sets used inside the main query.

CTEs improve code readability and can be reused within the 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