CTE with INSERT, UPDATE, DELETE (writable CTEs) in PostgreSQL

Introduction

Writable CTEs let you change data (insert, update, delete) inside a query. This helps organize complex changes step-by-step.

You want to update some rows and then use those updated rows immediately in the same query.

You need to insert new data and then select from it right away.

You want to delete some rows and then see what remains or do more changes.

You want to keep your data changes clear and easy to read by breaking them into parts.

You want to perform multiple data changes in one query safely.

Syntax

PostgreSQL

WITH cte_name AS (
  INSERT INTO table_name (column_list)
  VALUES (value_list)
  RETURNING *
)
SELECT * FROM cte_name;

Writable CTEs use INSERT, UPDATE, or DELETE inside the WITH clause.

Use RETURNING to get the changed rows and use them in the next query.

Examples

This updates salaries in Sales and shows the updated rows.

PostgreSQL

WITH updated AS (
  UPDATE employees
  SET salary = salary * 1.1
  WHERE department = 'Sales'
  RETURNING *
)
SELECT * FROM updated;

This inserts a new product and returns its details.

PostgreSQL

WITH inserted AS (
  INSERT INTO products (name, price)
  VALUES ('New Product', 20)
  RETURNING *
)
SELECT * FROM inserted;

This deletes old orders and counts how many were deleted.

PostgreSQL

WITH deleted AS (
  DELETE FROM orders
  WHERE order_date < '2023-01-01'
  RETURNING *
)
SELECT COUNT(*) FROM deleted;

Sample Program

This query adds 500 to the salary of all employees in the HR department and then shows their id, name, and new salary.

PostgreSQL

WITH updated_employees AS (
  UPDATE employees
  SET salary = salary + 500
  WHERE department = 'HR'
  RETURNING id, name, salary
)
SELECT * FROM updated_employees ORDER BY id;

Important Notes

Writable CTEs run the data change first, then the following SELECT uses the changed data.

Always use RETURNING to get the affected rows; otherwise, you can't use the changed data in the next step.

Writable CTEs help keep queries clean and easy to understand when doing multiple steps.

Summary

Writable CTEs let you insert, update, or delete data inside a WITH clause.

Use RETURNING to get the changed rows and use them immediately.

This makes complex data changes easier to write and read.

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