WITH clause syntax in PostgreSQL - Mini Project: Build & Apply

Using WITH Clause in PostgreSQL Queries

📖 Scenario: You are working with a sales database that tracks orders and customers. You want to organize your query by first selecting customers who have placed orders, then using that result to find their total order amounts.

🎯 Goal: Build a PostgreSQL query using the WITH clause to create a temporary result set of customers with orders, then use it to calculate total order amounts per customer.

📋 What You'll Learn

Create a WITH clause named customer_orders that selects customer_id and order_id from the orders table

Select customer_id and the sum of order_amount as total_amount from customer_orders joined with orders

Group the final results by customer_id

Order the results by total_amount in descending order

💡 Why This Matters

🌍 Real World

WITH clauses help organize complex queries by breaking them into smaller parts, making queries easier to read and maintain.

💼 Career

Database developers and analysts use WITH clauses to write clear and efficient SQL queries for reporting and data analysis.

1

Create the WITH clause named customer_orders

Write a WITH clause named customer_orders that selects customer_id and order_id from the orders table.

PostgreSQL

-- Write the WITH clause named customer_orders selecting customer_id and order_id from orders
-- Your code here

Hint

Use WITH customer_orders AS (SELECT customer_id, order_id FROM orders) to create the temporary result.

2

Select customer_id and sum of order_amount as total_amount

After the WITH clause, write a SELECT statement that selects customer_id and the sum of order_amount as total_amount from customer_orders joined with orders on order_id.

PostgreSQL

WITH customer_orders AS (
    SELECT customer_id, order_id FROM orders
)
-- Write SELECT statement to get customer_id and sum(order_amount) as total_amount
-- Your code here

Hint

Join customer_orders with orders on order_id and sum order_amount.

3

Group the results by customer_id

Add a GROUP BY clause to group the results by customer_id in your SELECT statement.

PostgreSQL

WITH customer_orders AS (
    SELECT customer_id, order_id FROM orders
)
SELECT co.customer_id, SUM(o.order_amount) AS total_amount
FROM customer_orders co
JOIN orders o ON co.order_id = o.order_id
-- Add GROUP BY customer_id
-- Your code here

Hint

Use GROUP BY co.customer_id to group the sums by customer.

4

Order the results by total_amount descending

Add an ORDER BY clause to sort the results by total_amount in descending order.

PostgreSQL

WITH customer_orders AS (
    SELECT customer_id, order_id FROM orders
)
SELECT co.customer_id, SUM(o.order_amount) AS total_amount
FROM customer_orders co
JOIN orders o ON co.order_id = o.order_id
GROUP BY co.customer_id
-- Add ORDER BY total_amount DESC
-- Your code here

Hint

Use ORDER BY total_amount DESC to sort from highest to lowest total.

Practice

1. What is the main purpose of the WITH clause in PostgreSQL?

easy

A. To create indexes on columns

B. To permanently store data in a new table

C. To create temporary named query results called CTEs

D. To delete rows from a table

Solution

1. Step 1: Understand the role of WITH clause

   The WITH clause defines Common Table Expressions (CTEs) which are temporary named result sets used within a query.

2. Step 2: Differentiate from other SQL commands

   It does not create permanent tables, delete data, or create indexes; it only helps organize complex queries.

3. Final Answer:

   To create temporary named query results called CTEs -> Option C

4. Quick Check:

   WITH clause = Temporary named queries [OK]

Hint: WITH clause defines temporary named queries (CTEs) [OK]

Common Mistakes:

• Thinking WITH creates permanent tables
• Confusing WITH with DELETE or CREATE INDEX
• Assuming WITH modifies data directly

2. Which of the following is the correct syntax to start a query using a WITH clause in PostgreSQL?

easy

A. WITH cte_name SELECT * FROM table_name AS cte_name;

B. WITH (SELECT * FROM table_name) AS cte_name SELECT * FROM cte_name;

C. WITH cte_name = SELECT * FROM table_name; SELECT * FROM cte_name;

D. WITH cte_name AS (SELECT * FROM table_name) SELECT * FROM cte_name;

Solution

1. Step 1: Recall correct WITH clause syntax

   The correct syntax is: WITH cte_name AS (subquery) followed by the main SELECT using cte_name.

2. Step 2: Check each option

   WITH cte_name AS (SELECT * FROM table_name) SELECT * FROM cte_name; matches the correct syntax. Options A, B, and D have misplaced keywords or wrong order.

3. Final Answer:

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

4. Quick Check:

   WITH cte_name AS (query) = correct syntax [OK]

Hint: WITH clause uses AS (subquery) syntax [OK]

Common Mistakes:

• Omitting AS keyword
• Placing SELECT before AS
• Using = instead of AS

3. Given the query:

WITH cte AS (SELECT id, name FROM users WHERE active = true) SELECT * FROM cte WHERE id < 5;

What will this query return?

medium

A. All users with id less than 5 regardless of active status

B. All users with active = true and id less than 5

C. All users with active = false and id less than 5

D. Syntax error due to incorrect WHERE clause

Solution

1. Step 1: Analyze the CTE definition

   The CTE named 'cte' selects users where active = true, so only active users are included.

2. Step 2: Analyze the main query filtering

   The main query selects from 'cte' where id < 5, so it filters active users with id less than 5.

3. Final Answer:

   All users with active = true and id less than 5 -> Option B

4. Quick Check:

   CTE filters active=true, main query filters id<5 [OK]

Hint: CTE filters first, main query filters second [OK]

Common Mistakes:

• Ignoring CTE filter and only applying main WHERE
• Assuming all users regardless of active status
• Thinking syntax error due to WHERE

4. Identify the error in the following query:

WITH cte1 AS (SELECT * FROM orders), cte2 (SELECT * FROM customers) SELECT * FROM cte1 JOIN cte2 ON cte1.customer_id = cte2.id;

medium

A. Missing AS keyword before the second CTE's subquery

B. Incorrect JOIN syntax between cte1 and cte2

C. CTEs cannot be used together in one WITH clause

D. SELECT statement missing after WITH clause

Solution

1. Step 1: Check syntax of multiple CTEs

   Multiple CTEs must be separated by commas, each with the syntax: name AS (subquery).

2. Step 2: Identify error in second CTE

   The second CTE 'cte2' is missing the AS keyword before its subquery.

3. Final Answer:

   Missing AS keyword before the second CTE's subquery -> Option A

4. Quick Check:

   Each CTE needs AS (subquery) [OK]

Hint: Each CTE must have AS before its query [OK]

Common Mistakes:

• Omitting AS keyword in CTE definition
• Misplacing commas between CTEs
• Incorrect JOIN syntax confusion

5. You want to write a query that uses two CTEs: one to get all active users, and another to get their orders. Which of the following is the correct way to write this using the WITH clause?

hard

A. WITH active_users AS (SELECT * FROM users WHERE active = true), user_orders AS (SELECT * FROM orders WHERE user_id IN (SELECT id FROM active_users)) SELECT active_users.name, user_orders.order_date FROM active_users JOIN user_orders ON active_users.id = user_orders.user_id;

B. WITH active_users (SELECT * FROM users WHERE active = true), user_orders AS (SELECT * FROM orders WHERE user_id IN (SELECT id FROM active_users)) SELECT * FROM active_users JOIN user_orders ON active_users.id = user_orders.user_id;

C. WITH active_users AS SELECT * FROM users WHERE active = true, user_orders AS (SELECT * FROM orders) SELECT * FROM active_users JOIN user_orders ON active_users.id = user_orders.user_id;

D. WITH active_users AS (SELECT * FROM users WHERE active = true) user_orders AS (SELECT * FROM orders) SELECT * FROM active_users JOIN user_orders ON active_users.id = user_orders.user_id;

Solution

1. Step 1: Verify correct syntax for multiple CTEs

   Multiple CTEs must be separated by commas, each defined as name AS (subquery).

2. Step 2: Check each option's syntax and logic

   WITH active_users AS (SELECT * FROM users WHERE active = true), user_orders AS (SELECT * FROM orders WHERE user_id IN (SELECT id FROM active_users)) SELECT active_users.name, user_orders.order_date FROM active_users JOIN user_orders ON active_users.id = user_orders.user_id; correctly uses AS and commas, and references active_users in user_orders. Options B, C, and D have syntax errors or missing commas.

3. Final Answer:

   WITH active_users AS (SELECT * FROM users WHERE active = true), user_orders AS (SELECT * FROM orders WHERE user_id IN (SELECT id FROM active_users)) SELECT active_users.name, user_orders.order_date FROM active_users JOIN user_orders ON active_users.id = user_orders.user_id; -> Option A

4. Quick Check:

   Multiple CTEs separated by commas with AS (query) [OK]

Hint: Separate multiple CTEs with commas and use AS (query) [OK]

Common Mistakes:

• Missing commas between CTEs
• Omitting AS keyword
• Not referencing CTEs properly in later queries