DELETE with RETURNING clause in PostgreSQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the DELETE statement do in SQL?

The DELETE statement removes rows from a table based on a specified condition.

beginner

What is the purpose of the RETURNING clause in a DELETE statement?

The RETURNING clause returns the deleted rows' data immediately after deletion, allowing you to see what was removed.

intermediate

Write a simple DELETE statement with RETURNING to remove users older than 30 and return their names.

DELETE FROM users WHERE age > 30 RETURNING name;

intermediate

Can the RETURNING clause be used with other SQL commands besides DELETE?

Yes, RETURNING can also be used with INSERT and UPDATE to return affected rows.

beginner

Why is RETURNING useful in applications?

RETURNING helps applications get immediate feedback on what data was changed without running a separate SELECT query.

What does the RETURNING clause do in a DELETE statement?

A Creates a backup of the table

B Prevents deletion of rows

C Updates rows instead of deleting

D Returns the deleted rows' data

The RETURNING clause returns the data of rows that were deleted.

Which SQL command can use the RETURNING clause?

A DELETE only

B CREATE only

C INSERT, UPDATE, DELETE

D SELECT only

RETURNING can be used with INSERT, UPDATE, and DELETE commands.

What will this query do? DELETE FROM orders WHERE id = 5 RETURNING *;

A Delete order with id 5 and return all its columns

B Delete all orders except id 5

C Return all orders without deleting

D Update order with id 5

It deletes the order with id 5 and returns all columns of that deleted row.

Is RETURNING clause standard SQL or PostgreSQL specific?

A PostgreSQL specific

B Standard SQL

C Only MySQL supports it

D Only Oracle supports it

RETURNING is a PostgreSQL-specific feature, not part of standard SQL.

Why might you prefer DELETE with RETURNING over DELETE followed by SELECT?

A It requires two queries

B It returns deleted data immediately in one query

C It is slower

D It does not delete rows

DELETE with RETURNING returns deleted rows immediately, avoiding a second query.

Explain how the DELETE statement with RETURNING clause works in PostgreSQL.

Think about deleting and seeing what was deleted at the same time.

Describe a real-life scenario where using DELETE with RETURNING would be helpful.

Imagine you want to confirm what you removed from a list.

Practice

1. What does the RETURNING clause do in a DELETE statement in PostgreSQL?

easy

A. It counts the number of rows in the table.

B. It prevents rows from being deleted.

C. It updates rows instead of deleting them.

D. It returns the rows that were deleted.

Solution

1. Step 1: Understand DELETE operation

   The DELETE statement removes rows from a table based on a condition.

2. Step 2: Role of RETURNING clause

   The RETURNING clause outputs the rows that were deleted, showing their data immediately.

3. Final Answer:

   It returns the rows that were deleted. -> Option D

4. Quick Check:

   RETURNING shows deleted rows [OK]

Hint: RETURNING shows deleted rows immediately [OK]

Common Mistakes:

• Thinking RETURNING prevents deletion
• Confusing RETURNING with UPDATE
• Assuming RETURNING counts rows

2. Which of the following is the correct syntax to delete rows from users where age < 18 and return all deleted rows?

easy

A. DELETE FROM users WHERE age < 18 RETURNING *;

B. DELETE users WHERE age < 18 RETURNING *;

C. DELETE FROM users RETURNING * WHERE age < 18;

D. DELETE FROM users WHERE age < 18 RETURNING all;

Solution

1. Step 1: Check DELETE syntax

   The correct DELETE syntax is: DELETE FROM table WHERE condition.

2. Step 2: Use RETURNING clause correctly

   RETURNING * must come after WHERE clause to return all columns of deleted rows.

3. Final Answer:

   DELETE FROM users WHERE age < 18 RETURNING *; -> Option A

4. Quick Check:

   Correct DELETE syntax with RETURNING * [OK]

Hint: DELETE FROM ... WHERE ... RETURNING *; is correct syntax [OK]

Common Mistakes:

• Omitting FROM keyword
• Placing RETURNING before WHERE
• Using RETURNING all instead of *

3. Given the table products with rows:
{id:1, name:'Pen', stock:10}, {id:2, name:'Pencil', stock:0}, {id:3, name:'Eraser', stock:5}
What will this query return?

DELETE FROM products WHERE stock = 0 RETURNING id, name;

medium

A. [{id:1, name:'Pen'}, {id:3, name:'Eraser'}]

B. []

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

D. Syntax error

Solution

1. Step 1: Identify rows matching condition

   Only the product with stock = 0 is {id:2, name:'Pencil'}.

2. Step 2: DELETE removes matching rows and RETURNING outputs specified columns

   The query deletes that row and returns its id and name.

3. Final Answer:

   [{id:2, name:'Pencil'}] -> Option C

4. Quick Check:

   Deleted rows with stock=0 returned [OK]

Hint: RETURNING shows deleted rows matching WHERE [OK]

Common Mistakes:

• Returning all rows instead of matching ones
• Expecting deleted rows to remain in table
• Confusing syntax causing errors

4. What is wrong with this query?

DELETE FROM orders RETURNING * WHERE status = 'cancelled';

medium

A. The WHERE clause must come before RETURNING.

B. RETURNING * is not allowed in DELETE.

C. Missing semicolon at the end.

D. DELETE cannot filter rows.

Solution

1. Step 1: Check clause order in DELETE

   In PostgreSQL, WHERE clause must come before RETURNING clause.

2. Step 2: Identify error in query

   The query places RETURNING * before WHERE, which is invalid syntax.

3. Final Answer:

   The WHERE clause must come before RETURNING. -> Option A

4. Quick Check:

   WHERE before RETURNING in DELETE [OK]

Hint: WHERE clause always before RETURNING [OK]

Common Mistakes:

• Placing RETURNING before WHERE
• Thinking RETURNING is disallowed
• Ignoring clause order rules

5. You want to delete all employees from staff who have salary < 30000 and return their id and name. Which query correctly does this and also returns the count of deleted rows?

hard

A. DELETE FROM staff WHERE salary < 30000 RETURNING id, name, COUNT(*);

B. WITH deleted AS (DELETE FROM staff WHERE salary < 30000 RETURNING id, name) SELECT COUNT(*) FROM deleted;

C. DELETE FROM staff RETURNING id, name WHERE salary < 30000; SELECT COUNT(*) FROM staff;

D. DELETE FROM staff WHERE salary < 30000; SELECT id, name FROM staff RETURNING COUNT(*);

Solution

1. Step 1: Delete rows with condition and return columns

   Using DELETE ... WHERE salary < 30000 RETURNING id, name returns deleted rows.

2. Step 2: Use WITH clause to capture deleted rows and count them

   WITH deleted AS (...) lets us select COUNT(*) from deleted rows in one query.

3. Final Answer:

   WITH deleted AS (DELETE FROM staff WHERE salary < 30000 RETURNING id, name) SELECT COUNT(*) FROM deleted; -> Option B

4. Quick Check:

   Use WITH + RETURNING to delete and count [OK]

Hint: Use WITH clause to count deleted rows with RETURNING [OK]

Common Mistakes:

• Trying to use COUNT(*) inside RETURNING
• Placing WHERE after RETURNING
• Separating DELETE and count without capturing deleted rows