DELETE with RETURNING clause in PostgreSQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to delete rows from the table and return the deleted rows.

PostgreSQL

DELETE FROM employees WHERE department = 'Sales' [1];

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

ARETURNING *

BOUTPUT *

CFETCH *

DRETURN *

Attempts:

3 left

💡 Hint

Common Mistakes

Using OUTPUT instead of RETURNING, which is not valid in PostgreSQL.

Using RETURN without RETURNING.

Using FETCH which is unrelated here.

The RETURNING clause in PostgreSQL allows you to return the deleted rows immediately after deletion.

2fill in blank

medium

Complete the code to delete employees older than 60 and return their names.

PostgreSQL

DELETE FROM employees WHERE age > 60 [1] name;

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

ARETURNING

BOUTPUT

CFETCH

DSELECT

Attempts:

3 left

💡 Hint

Common Mistakes

Using OUTPUT or FETCH which are not valid in PostgreSQL DELETE.

Using SELECT after DELETE which is invalid syntax.

Use RETURNING followed by the column name to get specific columns from deleted rows.

3fill in blank

hard

Fix the error in the DELETE statement to correctly return the deleted rows.

PostgreSQL

DELETE FROM orders WHERE order_date < '2023-01-01' [1];

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

AOUTPUT *

BRETURN *

CFETCH ALL

DRETURNING *

Attempts:

3 left

💡 Hint

Common Mistakes

Using OUTPUT which is SQL Server syntax.

Using RETURN which is invalid here.

Using FETCH which is unrelated.

PostgreSQL uses RETURNING * to return all columns of deleted rows.

4fill in blank

hard

Fill both blanks to delete products with zero stock and return their id and name.

PostgreSQL

DELETE FROM products WHERE stock = 0 [1] id, [2];

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

ARETURNING

BOUTPUT

Cname

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using OUTPUT instead of RETURNING.

Using '*' when specific columns are requested.

Omitting the comma between columns.

Use RETURNING followed by the columns you want to return, separated by commas.

5fill in blank

hard

Fill all three blanks to delete customers from 'Canada' and return their name and email.

PostgreSQL

DELETE FROM customers WHERE country = 'Canada' [1] [2], [3];

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

ARETURNING

Bid

Cemail

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Using OUTPUT instead of RETURNING.

Mixing up column order.

Forgetting commas between columns.

Use RETURNING followed by the columns you want to return, separated by commas.

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