CTE with INSERT, UPDATE, DELETE (writable CTEs) in PostgreSQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a writable CTE in PostgreSQL?

A writable CTE is a Common Table Expression that allows you to perform INSERT, UPDATE, or DELETE operations inside the CTE and then use the affected rows in the main query.

beginner

How do you use an UPDATE inside a writable CTE?

You write a CTE with the UPDATE statement, then select from it to see the updated rows or use them in further queries.

intermediate

Can you perform multiple writable CTEs in a single query?

Yes, you can chain multiple writable CTEs (INSERT, UPDATE, DELETE) in one query, and each can use the results of the previous ones.

intermediate

What is the benefit of using writable CTEs instead of separate statements?

Writable CTEs let you combine data modification and retrieval in one query, making your code cleaner and sometimes more efficient.

beginner

Give an example of a DELETE operation inside a writable CTE.

WITH deleted_rows AS (DELETE FROM table_name WHERE condition RETURNING *) SELECT * FROM deleted_rows;

What does a writable CTE allow you to do in PostgreSQL?

A Create temporary tables

B Only SELECT data inside a CTE

C Perform INSERT, UPDATE, or DELETE inside a CTE

D Run stored procedures

Writable CTEs allow data modification operations like INSERT, UPDATE, and DELETE inside the CTE.

Which keyword is used to get the affected rows from a writable CTE operation?

A INTO

B RETURNING

C FETCH

D OUTPUT

The RETURNING clause returns the rows affected by INSERT, UPDATE, or DELETE.

Can you chain multiple writable CTEs in one query?

A Yes, you can chain multiple writable CTEs

B No, only one writable CTE per query

C Only if they are INSERTs

D Only if they are SELECTs

PostgreSQL supports chaining multiple writable CTEs in a single query.

What is the main advantage of using writable CTEs?

A Make queries run faster always

B Automatically backup data

C Avoid using indexes

D Combine data modification and retrieval in one query

Writable CTEs let you modify data and use the results immediately in the same query.

Which statement is NOT valid inside a writable CTE?

A CREATE TABLE

B INSERT

C UPDATE

D SELECT

Writable CTEs support INSERT, UPDATE, DELETE, but not DDL statements like CREATE TABLE.

Explain how writable CTEs work in PostgreSQL and give an example of an UPDATE operation using a writable CTE.

Think about how you can update rows and then select them in one query.

Describe the benefits of using writable CTEs compared to running separate INSERT, UPDATE, or DELETE statements.

Consider how writable CTEs help organize queries.

Practice

1. What is the main purpose of using writable CTEs in PostgreSQL?

easy

A. To speed up SELECT queries by caching results

B. To create temporary tables that last beyond the current transaction

C. To perform INSERT, UPDATE, or DELETE operations inside a WITH clause

D. To define functions that return multiple rows

Solution

1. Step 1: Understand writable CTEs

   Writable CTEs allow data modification commands like INSERT, UPDATE, or DELETE inside a WITH clause.

2. Step 2: Compare options

   Options B, C, and D describe other features unrelated to writable CTEs.

3. Final Answer:

   To perform INSERT, UPDATE, or DELETE operations inside a WITH clause -> Option C

4. Quick Check:

   Writable CTEs = Data modification in WITH [OK]

Hint: Writable CTEs = data changes inside WITH clause [OK]

Common Mistakes:

• Confusing writable CTEs with temporary tables
• Thinking writable CTEs speed up SELECT queries
• Mixing writable CTEs with function definitions

2. Which of the following is the correct syntax to update rows using a writable CTE in PostgreSQL?

easy

A. UPDATE employees WITH (salary = salary * 1.1) RETURNING *;

B. WITH updated AS (UPDATE employees SET salary = salary * 1.1) RETURNING *;

C. WITH updated AS (SELECT * FROM employees WHERE salary > 5000) UPDATE updated SET salary = salary * 1.1;

D. WITH updated AS (UPDATE employees SET salary = salary * 1.1 RETURNING *) SELECT * FROM updated;

Solution

1. Step 1: Review writable CTE update syntax

   The correct syntax uses WITH updated AS (UPDATE ... RETURNING *) followed by a SELECT from updated.

2. Step 2: Analyze options

   WITH updated AS (UPDATE employees SET salary = salary * 1.1 RETURNING *) SELECT * FROM updated; matches the correct syntax. Options A, C, and D have syntax errors or misuse RETURNING.

3. Final Answer:

   WITH updated AS (UPDATE employees SET salary = salary * 1.1 RETURNING *) SELECT * FROM updated; -> Option D

4. Quick Check:

   Writable CTE update = WITH + UPDATE + RETURNING + SELECT [OK]

Hint: Use WITH (UPDATE ... RETURNING *) then SELECT [OK]

Common Mistakes:

• Omitting RETURNING clause in writable CTE
• Trying to UPDATE inside WITH without SELECT
• Misplacing RETURNING outside the CTE

3. Given the table products(id, name, stock) with rows: (1, 'Pen', 10), (2, 'Pencil', 5), what will be the result of this query?

WITH updated AS (UPDATE products SET stock = stock + 10 WHERE name = 'Pen' RETURNING *) SELECT * FROM updated;

medium

A. [{id:1, name:'Pen', stock:20}]

B. [{id:1, name:'Pen', stock:10}]

C. [{id:2, name:'Pencil', stock:15}]

D. Empty result set

Solution

1. Step 1: Identify updated rows

   The UPDATE increases stock by 10 only for product with name 'Pen' (id=1), changing stock from 10 to 20.

2. Step 2: Understand RETURNING and SELECT

   The RETURNING * returns the updated row, and the SELECT outputs it.

3. Final Answer:

   [{id:1, name:'Pen', stock:20}] -> Option A

4. Quick Check:

   Updated Pen stock = 10 + 10 = 20 [OK]

Hint: RETURNING shows updated rows immediately [OK]

Common Mistakes:

• Assuming all rows are updated
• Ignoring RETURNING clause output
• Confusing stock before and after update

4. Identify why this writable CTE query returns an empty result set even if matching rows exist:

WITH deleted AS (DELETE FROM orders WHERE status = 'cancelled') SELECT * FROM deleted;

medium

A. The WHERE clause syntax is incorrect

B. Missing RETURNING clause in DELETE statement

C. SELECT * FROM deleted should be SELECT COUNT(*)

D. DELETE cannot be used inside WITH clause

Solution

1. Step 1: Check DELETE in writable CTE

   DELETE inside WITH must include RETURNING to produce rows for the outer SELECT.

2. Step 2: Analyze the query

   The query lacks RETURNING, so 'deleted' has no rows to select from, resulting in an empty result set.

3. Final Answer:

   Missing RETURNING clause in DELETE statement -> Option B

4. Quick Check:

   Writable CTE DELETE needs RETURNING [OK]

Hint: Always add RETURNING with DELETE in writable CTEs [OK]

Common Mistakes:

• Omitting RETURNING after DELETE in CTE
• Thinking DELETE can't be in WITH clause
• Misunderstanding SELECT after writable CTE

5. You want to insert new users and immediately update their status to 'active' using writable CTEs. Which query correctly does this in one statement?

hard

A. WITH ins AS (INSERT INTO users(name) VALUES ('Alice'), ('Bob') RETURNING id, name) UPDATE users SET status = 'active' FROM ins WHERE users.id = ins.id RETURNING users.*;

B. INSERT INTO users(name) VALUES ('Alice'), ('Bob'); WITH upd AS (UPDATE users SET status = 'active' WHERE name IN ('Alice', 'Bob') RETURNING *) SELECT * FROM upd;

C. WITH ins AS (INSERT INTO users(name) VALUES ('Alice'), ('Bob')) UPDATE users SET status = 'active' WHERE id IN (SELECT id FROM ins);

D. WITH ins AS (INSERT INTO users(name) VALUES ('Alice'), ('Bob')) UPDATE ins SET status = 'active' WHERE name IN ('Alice', 'Bob');

Solution

1. Step 1: Insert and capture new rows

   Use a writable CTE to insert and RETURNING id, name to identify new users.

2. Step 2: Update using CTE results

   Update users joining with inserted rows on id, setting status to 'active', and RETURNING updated rows.

3. Step 3: Analyze options

   WITH ins AS (INSERT INTO users(name) VALUES ('Alice'), ('Bob') RETURNING id, name) UPDATE users SET status = 'active' FROM ins WHERE users.id = ins.id RETURNING users.*; correctly chains insert and update in one statement with proper RETURNING and join. INSERT INTO users(name) VALUES ('Alice'), ('Bob'); WITH upd AS (UPDATE users SET status = 'active' WHERE name IN ('Alice', 'Bob') RETURNING *) SELECT * FROM upd; separates insert and update into multiple statements. WITH ins AS (INSERT INTO users(name) VALUES ('Alice'), ('Bob')) UPDATE users SET status = 'active' WHERE id IN (SELECT id FROM ins); lacks RETURNING so CTE returns no rows and subquery updates nothing. WITH ins AS (INSERT INTO users(name) VALUES ('Alice'), ('Bob')) UPDATE ins SET status = 'active' WHERE name IN ('Alice', 'Bob'); incorrectly attempts to UPDATE the CTE which is invalid.

4. Final Answer:

   WITH ins AS (INSERT INTO users(name) VALUES ('Alice'), ('Bob') RETURNING id, name) UPDATE users SET status = 'active' FROM ins WHERE users.id = ins.id RETURNING users.*; -> Option A

5. Quick Check:

   Writable CTE insert + update with join = WITH ins AS (INSERT INTO users(name) VALUES ('Alice'), ('Bob') RETURNING id, name) UPDATE users SET status = 'active' FROM ins WHERE users.id = ins.id RETURNING users.*; [OK]

Hint: Chain INSERT and UPDATE with RETURNING and FROM join [OK]

Common Mistakes:

• Omitting RETURNING clause before using CTE in subquery
• Splitting insert and update into separate statements
• Omitting RETURNING to get updated rows