DELETE with RETURNING clause in PostgreSQL - Step-by-Step Execution

Concept Flow - DELETE with RETURNING clause

Start DELETE command

↓

Identify rows matching condition

↓

Delete those rows from table

↓

RETURNING clause fetches deleted rows

↓

Output deleted rows as result

↓

End

The DELETE command finds rows matching a condition, removes them, and the RETURNING clause outputs the deleted rows as a result.

Execution Sample

PostgreSQL

DELETE FROM employees
WHERE department = 'Sales'
RETURNING id, name;

Deletes employees in the Sales department and returns their id and name.

Execution Table

Step	Action	Rows Matched	Rows Deleted	RETURNING Output
1	Start DELETE command	-	-	-
2	Find rows where department = 'Sales'	3 rows found	-	-
3	Delete matched rows	3 rows matched	3 rows deleted	-
4	RETURNING outputs deleted rows' id and name	-	-	[{"id": 2, "name": "Alice"}, {"id": 5, "name": "Bob"}, {"id": 7, "name": "Carol"}]
5	End of DELETE with RETURNING	-	-	-

💡 All rows matching condition deleted and returned, operation complete.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
rows_matched	0	3	3	3	3
rows_deleted	0	0	3	3	3
returning_output	[]	[]	[]	[{"id":2,"name":"Alice"},{"id":5,"name":"Bob"},{"id":7,"name":"Carol"}]	[{"id":2,"name":"Alice"},{"id":5,"name":"Bob"},{"id":7,"name":"Carol"}]

Key Moments - 3 Insights

Why do we see the deleted rows in the output?

Because the RETURNING clause tells PostgreSQL to output the deleted rows' specified columns after deletion, as shown in step 4 of the execution_table.

Does the DELETE command remove rows before or after RETURNING outputs them?

Rows are deleted first (step 3), then RETURNING outputs the deleted rows (step 4), so the output reflects the rows just removed.

What happens if no rows match the condition?

No rows are deleted or returned; the RETURNING output will be empty, as no rows matched in step 2.

Visual Quiz - 3 Questions

Look at the execution_table, how many rows were deleted at step 3?

A 3 rows

B 0 rows

C 5 rows

D 2 rows

💡 Hint

Check the 'Rows Deleted' column at step 3 in the execution_table.

At which step does the RETURNING clause output the deleted rows?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Look at the 'RETURNING Output' column in the execution_table.

If the WHERE condition matched zero rows, what would be the value of 'rows_deleted' after step 3?

A 3

B 1

C 0

D Undefined

💡 Hint

Refer to the variable_tracker row for 'rows_deleted' and consider what happens if no rows match.

Concept Snapshot

DELETE FROM table_name
WHERE condition
RETURNING column1, column2;

- Deletes rows matching condition
- RETURNING outputs deleted rows
- Useful to see what was removed
- Works only in PostgreSQL

Full Transcript

This visual execution shows how the DELETE command with RETURNING clause works in PostgreSQL. First, the command starts and finds rows matching the WHERE condition. Then, those rows are deleted from the table. After deletion, the RETURNING clause outputs the specified columns of the deleted rows as the result. The execution table tracks each step, showing how many rows matched, were deleted, and what the RETURNING clause outputs. The variable tracker shows how key variables change after each step. Key moments clarify common confusions, such as why deleted rows appear in output and the order of deletion and returning. The quiz tests understanding of these steps. The snapshot summarizes the syntax and behavior for quick reference.

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