CTE with INSERT, UPDATE, DELETE (writable CTEs) in PostgreSQL - Mini Project: Build & Apply

Managing Employee Records with Writable CTEs in PostgreSQL

📖 Scenario: You are managing a small company's employee database. You need to add new employees, update existing employee salaries, and remove employees who have left the company. Using writable Common Table Expressions (CTEs) in PostgreSQL will help you perform these operations cleanly and efficiently.

🎯 Goal: Build a PostgreSQL script that uses writable CTEs to insert a new employee, update an existing employee's salary, and delete an employee who left the company, all in one query.

📋 What You'll Learn

Create a table called employees with columns id (integer primary key), name (text), and salary (integer).

Insert initial employee data into the employees table.

Define a writable CTE to insert a new employee.

Define a writable CTE to update the salary of an existing employee.

Define a writable CTE to delete an employee by id.

Select all remaining employees after the changes.

💡 Why This Matters

🌍 Real World

Writable CTEs help database administrators and developers perform multiple data modifications in a single, readable query, improving maintainability and reducing errors.

💼 Career

Understanding writable CTEs is valuable for roles like database developer, data engineer, and backend developer working with PostgreSQL or similar SQL databases.

1

Create the employees table and insert initial data

Create a table called employees with columns id as primary key integer, name as text, and salary as integer. Then insert these exact rows: (1, 'Alice', 50000), (2, 'Bob', 60000), and (3, 'Charlie', 55000).

PostgreSQL

-- Create the employees table and insert initial data
-- Your code here

Hint

Use CREATE TABLE to define the structure, then INSERT INTO to add the rows.

2

Add a writable CTE to insert a new employee

Add a writable CTE named new_employee that inserts a new employee with id = 4, name = 'Diana', and salary = 62000 into the employees table.

PostgreSQL

CREATE TABLE employees (
    id INTEGER PRIMARY KEY,
    name TEXT,
    salary INTEGER
);

INSERT INTO employees (id, name, salary) VALUES
    (1, 'Alice', 50000),
    (2, 'Bob', 60000),
    (3, 'Charlie', 55000);

WITH new_employee AS (
    -- Insert new employee here
    -- Your code here
)

Hint

Use WITH new_employee AS (INSERT INTO ... RETURNING *) to create the writable CTE.

3

Add writable CTEs to update and delete employees

Extend the query by adding two more writable CTEs: updated_salary that updates Bob's salary to 65000, and deleted_employee that deletes the employee with id = 3 (Charlie). Use UPDATE and DELETE statements with RETURNING * inside the CTEs.

PostgreSQL

CREATE TABLE employees (
    id INTEGER PRIMARY KEY,
    name TEXT,
    salary INTEGER
);

INSERT INTO employees (id, name, salary) VALUES
    (1, 'Alice', 50000),
    (2, 'Bob', 60000),
    (3, 'Charlie', 55000);

WITH new_employee AS (
    INSERT INTO employees (id, name, salary)
    VALUES (4, 'Diana', 62000)
    RETURNING *
),
updated_salary AS (
    -- Update Bob's salary here
    -- Your code here
),
deleted_employee AS (
    -- Delete Charlie here
    -- Your code here
)

Hint

Use UPDATE employees SET salary = 65000 WHERE name = 'Bob' RETURNING * and DELETE FROM employees WHERE id = 3 RETURNING * inside CTEs.

4

Select all employees after the changes

Complete the query by adding a final SELECT statement that retrieves all columns from the employees table after the insert, update, and delete operations.

PostgreSQL

CREATE TABLE employees (
    id INTEGER PRIMARY KEY,
    name TEXT,
    salary INTEGER
);

INSERT INTO employees (id, name, salary) VALUES
    (1, 'Alice', 50000),
    (2, 'Bob', 60000),
    (3, 'Charlie', 55000);

WITH new_employee AS (
    INSERT INTO employees (id, name, salary)
    VALUES (4, 'Diana', 62000)
    RETURNING *
),
updated_salary AS (
    UPDATE employees
    SET salary = 65000
    WHERE name = 'Bob'
    RETURNING *
),
deleted_employee AS (
    DELETE FROM employees
    WHERE id = 3
    RETURNING *
)

SELECT * FROM employees;
-- Your code here

Hint

Use SELECT * FROM employees; after the CTEs to see the final table state.

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