UPDATE with RETURNING clause in PostgreSQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of UPDATE with RETURNING clause

Given the table employees with columns id, name, and salary, what is the output of this query?

UPDATE employees SET salary = salary + 1000 WHERE id = 3 RETURNING id, salary;

PostgreSQL

UPDATE employees SET salary = salary + 1000 WHERE id = 3 RETURNING id, salary;

A [{ "id": 3, "salary": 6000 }]

B [{ "id": 3, "salary": 5000 }]

C []

D SyntaxError

💡 Hint

The RETURNING clause outputs the updated rows after the UPDATE.

The query increases salary by 1000 for employee with id 3. If original salary was 5000, new salary is 6000. RETURNING shows the updated row.

🧠 Conceptual

intermediate

Purpose of RETURNING clause in UPDATE

What is the main purpose of using the RETURNING clause in an UPDATE statement in PostgreSQL?

A To return the rows that were updated with their new values

B To rollback the update if a condition fails

C To update multiple tables at once

D To create a backup of the updated rows

💡 Hint

Think about what you want to see immediately after updating rows.

RETURNING lets you see the updated rows and their new values without running a separate SELECT.

📝 Syntax

advanced

Identify the syntax error in UPDATE with RETURNING

Which of the following UPDATE statements with RETURNING clause is syntactically correct?

A UPDATE products SET price = price * 1.1 RETURNING;

B UPDATE products SET price = price * 1.1 RETURNING id, price;

C UPDATE products SET price = price * 1.1 RETURNING id price;

D UPDATE products SET price = price * 1.1 RETURNING id, price

💡 Hint

RETURNING must specify columns or * and end with semicolon.

Option B correctly lists columns and ends with semicolon. B misses columns. C misses comma. D misses semicolon.

❓ optimization

advanced

Optimizing UPDATE with RETURNING for multiple rows

You want to update salaries for all employees in department 5 by 10% and get their new salaries. Which query is the most efficient?

A UPDATE employees SET salary = salary * 1.1 WHERE department_id = 5; SELECT id, salary FROM employees WHERE department_id = 5;

B UPDATE employees SET salary = salary * 1.1 RETURNING id, salary WHERE department_id = 5;

C SELECT id, salary FROM employees WHERE department_id = 5; UPDATE employees SET salary = salary * 1.1 WHERE department_id = 5;

D UPDATE employees SET salary = salary * 1.1 WHERE department_id = 5 RETURNING id, salary;

💡 Hint

Consider how many queries run and how data is returned.

Option D updates and returns updated rows in one query. A runs two queries. C returns old salaries before update. D has syntax error.

🔧 Debug

expert

Error caused by incorrect RETURNING usage

What error will this query produce?

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

A No error, query runs successfully

B RuntimeError: invalid date format

C SyntaxError: missing columns after RETURNING

D TypeError: wrong data type in SET clause

💡 Hint

RETURNING must be followed by columns or *.

RETURNING clause requires columns or * after it. Here it is empty, causing syntax error.

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