CTE with INSERT, UPDATE, DELETE (writable CTEs) in PostgreSQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to insert a new user into the users table using a writable CTE.

PostgreSQL

WITH new_user AS (INSERT INTO users (name, email) VALUES ('Alice', '[e-mail]') RETURNING [1]) SELECT * FROM new_user;

Drag options to blanks, or click blank then click option'

A*

Bid

Cname

Demail

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting RETURNING clause causes no rows to be returned.

Using RETURNING without specifying columns returns an error in some databases.

The RETURNING * clause returns all columns of the inserted row, which is useful to see the full inserted record.

2fill in blank

medium

Complete the code to update the email of a user with id 5 using a writable CTE.

PostgreSQL

WITH updated_user AS (UPDATE users SET email = '[e-mail]' WHERE id = 5 RETURNING [1]) SELECT * FROM updated_user;

Drag options to blanks, or click blank then click option'

Aemail

Bname

C*

Did

Attempts:

3 left

💡 Hint

Common Mistakes

Not using RETURNING causes no rows to be returned.

Returning only one column may miss other updated data.

Using RETURNING * returns all columns of the updated row, showing the full updated record.

3fill in blank

hard

Fix the error in the DELETE statement to return the deleted rows using a writable CTE.

PostgreSQL

WITH deleted_users AS (DELETE FROM users WHERE last_login < '2023-01-01' RETURNING [1]) SELECT * FROM deleted_users;

Drag options to blanks, or click blank then click option'

Alast_login

Bid

Cemail

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting RETURNING clause causes no output.

Returning only one column may not show full deleted data.

Using RETURNING * returns all columns of the deleted rows, which is useful to see what was removed.

4fill in blank

hard

Fill both blanks to insert multiple products and return their ids and names.

PostgreSQL

WITH inserted_products AS (INSERT INTO products (name, price) VALUES ('Pen', 1.5), ('Notebook', 3.0) RETURNING [1], [2]) SELECT * FROM inserted_products;

Drag options to blanks, or click blank then click option'

Aid

Bname

Cprice

Ddescription

Attempts:

3 left

💡 Hint

Common Mistakes

Returning columns not in the table causes errors.

Forgetting commas between columns in RETURNING.

Returning id and name shows the unique identifier and the product name of inserted rows.

5fill in blank

hard

Fill all three blanks to update user status, return id, status, and email of updated users.

PostgreSQL

WITH updated_status AS (UPDATE users SET status = 'active' WHERE last_login > '2024-01-01' RETURNING [1], [2], [3]) SELECT * FROM updated_status;

Drag options to blanks, or click blank then click option'

Aid

Bstatus

Cemail

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Returning columns not updated or irrelevant.

Missing commas between columns in RETURNING.

Returning id, status, and email shows key details of the updated users.

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