CASE expression in PostgreSQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of CASE expression with simple conditions

What is the output of the following SQL query?

SELECT
  product_id,
  CASE
    WHEN price < 50 THEN 'Cheap'
    WHEN price BETWEEN 50 AND 100 THEN 'Moderate'
    ELSE 'Expensive'
  END AS price_category
FROM products
ORDER BY product_id
LIMIT 3;

PostgreSQL

CREATE TEMP TABLE products (product_id INT, price INT);
INSERT INTO products VALUES (1, 30), (2, 75), (3, 150);
SELECT
  product_id,
  CASE
    WHEN price < 50 THEN 'Cheap'
    WHEN price BETWEEN 50 AND 100 THEN 'Moderate'
    ELSE 'Expensive'
  END AS price_category
FROM products
ORDER BY product_id
LIMIT 3;

A [{"product_id":1,"price_category":"Cheap"},{"product_id":2,"price_category":"Moderate"},{"product_id":3,"price_category":"Expensive"}]

B [{"product_id":1,"price_category":"Expensive"},{"product_id":2,"price_category":"Cheap"},{"product_id":3,"price_category":"Moderate"}]

C [{"product_id":1,"price_category":"Moderate"},{"product_id":2,"price_category":"Moderate"},{"product_id":3,"price_category":"Cheap"}]

D [{"product_id":1,"price_category":null},{"product_id":2,"price_category":null},{"product_id":3,"price_category":null}]

💡 Hint

Think about the price ranges and how CASE matches conditions in order.

The CASE expression checks conditions in order. For product_id 1, price 30 is less than 50, so 'Cheap'. For product_id 2, price 75 is between 50 and 100, so 'Moderate'. For product_id 3, price 150 is greater than 100, so 'Expensive'.

📝 Syntax

intermediate

Identify the syntax error in CASE expression

Which option contains a syntax error in the CASE expression?

SELECT
  employee_id,
  CASE salary
    WHEN > 50000 THEN 'High'
    WHEN <= 50000 THEN 'Low'
  END AS salary_level
FROM employees;

A CASE WHEN salary > 50000 THEN 'High' WHEN salary <= 50000 THEN 'Low' END

B CASE salary WHEN > 50000 THEN 'High' WHEN <= 50000 THEN 'Low' END

C CASE salary WHEN 50000 THEN 'Equal' ELSE 'Other' END

D CASE WHEN salary = 50000 THEN 'Equal' ELSE 'Other' END

💡 Hint

Check how the simple CASE expression compares values.

In option B, the syntax 'WHEN > 50000' is invalid because simple CASE compares the expression to a value, not a condition. Conditions require the searched CASE syntax as in option B.

❓ query_result

advanced

Output of nested CASE expressions

What is the output of this query?

SELECT
  order_id,
  CASE
    WHEN status = 'shipped' THEN
      CASE
        WHEN shipped_date <= order_date + INTERVAL '3 days' THEN 'On time'
        ELSE 'Late'
      END
    ELSE 'Pending'
  END AS delivery_status
FROM orders
ORDER BY order_id
LIMIT 3;

PostgreSQL

CREATE TEMP TABLE orders (order_id INT, status TEXT, order_date DATE, shipped_date DATE);
INSERT INTO orders VALUES
(1, 'shipped', '2024-01-01', '2024-01-03'),
(2, 'shipped', '2024-01-01', '2024-01-06'),
(3, 'processing', '2024-01-01', NULL);
SELECT
  order_id,
  CASE
    WHEN status = 'shipped' THEN
      CASE
        WHEN shipped_date <= order_date + INTERVAL '3 days' THEN 'On time'
        ELSE 'Late'
      END
    ELSE 'Pending'
  END AS delivery_status
FROM orders
ORDER BY order_id
LIMIT 3;

A [{"order_id":1,"delivery_status":"Pending"},{"order_id":2,"delivery_status":"Pending"},{"order_id":3,"delivery_status":"Pending"}]

B [{"order_id":1,"delivery_status":null},{"order_id":2,"delivery_status":null},{"order_id":3,"delivery_status":null}]

C [{"order_id":1,"delivery_status":"Late"},{"order_id":2,"delivery_status":"On time"},{"order_id":3,"delivery_status":"Pending"}]

D [{"order_id":1,"delivery_status":"On time"},{"order_id":2,"delivery_status":"Late"},{"order_id":3,"delivery_status":"Pending"}]

💡 Hint

Check the nested CASE logic and date comparisons carefully.

Order 1 shipped within 2 days (<= 3 days), so 'On time'. Order 2 shipped after 5 days (> 3 days), so 'Late'. Order 3 is not shipped, so 'Pending'.

🔧 Debug

advanced

Find the logical error in CASE expression

The following query is intended to classify ages into groups but produces incorrect results. What is the logical error?

SELECT
  person_id,
  CASE
    WHEN age > 18 THEN 'Adult'
    WHEN age > 12 THEN 'Teen'
    ELSE 'Child'
  END AS age_group
FROM people;

A The order of WHEN conditions causes all ages above 12 to be classified as 'Adult'.

B The ELSE clause should be first to catch all ages.

C The CASE expression is missing an END keyword.

D The age comparisons should use BETWEEN instead of >.

💡 Hint

Think about how CASE evaluates conditions in order.

Since WHEN age > 18 comes first, ages above 18 are 'Adult'. But ages between 13 and 18 also satisfy age > 12, but that condition is never reached because the first WHEN is checked first. The order should be reversed to check smaller ranges first.

❓ optimization

expert

Optimize CASE expression for performance

Which option is the most efficient way to write this CASE expression for a large table?

SELECT
  user_id,
  CASE
    WHEN score >= 90 THEN 'A'
    WHEN score >= 80 THEN 'B'
    WHEN score >= 70 THEN 'C'
    WHEN score >= 60 THEN 'D'
    ELSE 'F'
  END AS grade
FROM exam_results;

A Use nested CASE expressions for each grade level.

B Use multiple OR conditions in a single WHEN clause to reduce checks.

C Use a derived table with ranges and join on score to get grade.

D Use a CASE expression with ordered WHEN conditions as shown.

💡 Hint

Consider how indexing and joins can improve performance over many CASE checks.

Using a derived table with score ranges and joining on score allows the database to use indexes and reduces repeated condition checks, improving performance on large datasets compared to multiple CASE WHEN checks.