Why UPDATE with RETURNING clause in PostgreSQL? - Purpose & Use Cases

The Big Idea

What if you could update your data and instantly see the results without extra searching?

The Scenario

Imagine you have a big list of customer orders in a spreadsheet. You want to change the status of some orders and then see exactly which ones changed and what their new status is. Doing this by hand means updating each order one by one and then searching again to find the updated rows.

The Problem

Manually updating and then searching again is slow and easy to mess up. You might forget which orders you changed or accidentally check the wrong rows. It wastes time and can cause mistakes, especially when working with many records.

The Solution

The UPDATE with RETURNING clause lets you update rows and immediately get back the changed data in one step. This means you don't have to run a separate search after updating. It saves time and reduces errors by showing exactly what was updated right away.

Before vs After

✗ Before

UPDATE orders SET status = 'shipped' WHERE order_date < '2024-01-01';
SELECT * FROM orders WHERE status = 'shipped' AND order_date < '2024-01-01';

✓ After

UPDATE orders SET status = 'shipped' WHERE order_date < '2024-01-01' RETURNING order_id, status;

What It Enables

This lets you instantly see which records changed and their new values, making your updates clear and efficient.

Real Life Example

A store manager updates all pending orders before a holiday and immediately gets a list of those orders now marked as shipped, ready to inform customers.

Key Takeaways

Updating and retrieving changed data happens in one step.

Reduces errors by showing exactly what was updated.

Saves time by avoiding extra queries after updates.

Practice

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

easy

A. It returns the updated rows immediately after the update.

B. It deletes the rows after updating them.

C. It creates a backup of the updated rows.

D. It prevents the update from happening.

Solution

1. Step 1: Understand the purpose of UPDATE

   The UPDATE statement changes data in existing rows of a table.

2. Step 2: Role of RETURNING clause

   The RETURNING clause immediately outputs the rows that were updated, showing their new values.

3. Final Answer:

   It returns the updated rows immediately after the update. -> Option A

4. Quick Check:

   RETURNING shows updated rows [OK]

Hint: RETURNING shows updated rows right after update [OK]

Common Mistakes:

• Thinking RETURNING deletes rows
• Assuming RETURNING creates backups
• Believing RETURNING stops the update

2. Which of the following is the correct syntax to update a table and return the updated rows in PostgreSQL?

easy

A. UPDATE table_name SET column1 = value1 WHERE condition RETURNING *;

B. UPDATE table_name SET column1 = value1 RETURN * WHERE condition;

C. UPDATE table_name RETURNING * SET column1 = value1 WHERE condition;

D. UPDATE table_name SET column1 = value1 WHERE condition SHOW RETURNING *;

Solution

1. Step 1: Identify correct UPDATE syntax

   The UPDATE statement must have SET clause before WHERE clause.

2. Step 2: Position of RETURNING clause

   RETURNING clause comes after WHERE clause to output updated rows.

3. Final Answer:

   UPDATE table_name SET column1 = value1 WHERE condition RETURNING *; -> Option A

4. Quick Check:

   RETURNING follows WHERE in UPDATE [OK]

Hint: RETURNING always comes after WHERE clause [OK]

Common Mistakes:

• Placing RETURNING before SET clause
• Using SHOW RETURNING which is invalid
• Putting RETURNING between SET and WHERE

3. Given the table employees with columns id, salary, and department, what will this query return?

UPDATE employees SET salary = salary + 500 WHERE department = 'Sales' RETURNING id, salary;

medium

A. Only the number of rows updated, no data returned.

B. All employees' ids and salaries regardless of department.

C. All employees' ids and new salaries in the Sales department after the raise.

D. An error because RETURNING cannot specify columns.

Solution

1. Step 1: Understand the UPDATE condition

   The query updates salary by adding 500 only for employees in the Sales department.

2. Step 2: Effect of RETURNING clause

   RETURNING id, salary outputs the id and updated salary of only those employees updated.

3. Final Answer:

   All employees' ids and new salaries in the Sales department after the raise. -> Option C

4. Quick Check:

   RETURNING outputs updated rows only [OK]

Hint: RETURNING shows only updated rows, not all rows [OK]

Common Mistakes:

• Thinking all rows are returned
• Expecting only count of updated rows
• Believing RETURNING cannot specify columns

4. What is wrong with this query?

UPDATE products RETURNING * SET price = price * 1.1 WHERE category = 'Books';

medium

A. The price calculation is invalid syntax.

B. The RETURNING clause is placed before SET, causing a syntax error.

C. The WHERE clause is missing.

D. RETURNING cannot be used with UPDATE.

Solution

1. Step 1: Check clause order in UPDATE

   UPDATE syntax requires SET before WHERE and RETURNING last.

2. Step 2: Identify error in query

   RETURNING appears before SET, which is invalid and causes syntax error.

3. Final Answer:

   The RETURNING clause is placed before SET, causing a syntax error. -> Option B

4. Quick Check:

   RETURNING must come after WHERE and SET [OK]

Hint: RETURNING always comes last after WHERE [OK]

Common Mistakes:

• Placing RETURNING before SET
• Omitting WHERE clause when needed
• Misordering clauses in UPDATE

5. You want to increase the salary by 10% for employees in the 'Engineering' department and immediately get their id, old_salary, and new_salary. Which query correctly does this using UPDATE with RETURNING?

hard

A. UPDATE employees SET salary = salary * 1.1 WHERE department = 'Engineering' RETURNING id, salary, salary / 1.1 AS old_salary;

B. UPDATE employees SET salary = salary * 1.1 WHERE department = 'Engineering' RETURNING id, salary, salary * 1.1 AS old_salary;

C. UPDATE employees SET salary = salary * 1.1 WHERE department = 'Engineering' RETURNING id, salary AS old_salary, salary * 1.1 AS new_salary;

D. UPDATE employees SET salary = salary * 1.1 WHERE department = 'Engineering' RETURNING id, salary AS new_salary, salary / 1.1 AS old_salary;

Solution

1. Step 1: Understand the salary update

   Salary is multiplied by 1.1 to increase by 10%, so new salary is salary * 1.1.

2. Step 2: Calculate old salary in RETURNING

   After update, salary column shows new salary. To get old salary, divide new salary by 1.1.

3. Step 3: Check aliasing in RETURNING

   RETURNING must show id, new_salary (salary), and old_salary (salary / 1.1) with correct aliases.

4. Final Answer:

   UPDATE employees SET salary = salary * 1.1 WHERE department = 'Engineering' RETURNING id, salary AS new_salary, salary / 1.1 AS old_salary; -> Option D

5. Quick Check:

   Old salary = new salary / 1.1 [OK]

Hint: Use RETURNING with expressions and aliases for old and new values [OK]

Common Mistakes:

• Confusing old and new salary calculations
• Wrong aliasing in RETURNING clause
• Using expressions incorrectly in RETURNING