CASE WHEN expression in MySQL

Introduction

The CASE WHEN expression helps you choose different results based on conditions, like making decisions in your data.

You want to label data based on values, like marking scores as 'Pass' or 'Fail'.

You need to create new categories from existing data without changing the original table.

You want to replace complex multiple IF statements with a cleaner way to check conditions.

You want to show different text or numbers depending on the data in each row.

You want to summarize data with custom groups in a query result.

Syntax

MySQL

CASE
  WHEN condition1 THEN result1
  WHEN condition2 THEN result2
  ...
  ELSE default_result
END

The CASE expression checks each WHEN condition in order and returns the result for the first true condition.

The ELSE part is optional; if no conditions match and ELSE is missing, the result is NULL.

Examples

This example labels each exam score as 'Pass' if 60 or more, otherwise 'Fail'.

MySQL

SELECT
  CASE
    WHEN score >= 60 THEN 'Pass'
    ELSE 'Fail'
  END AS result
FROM exams;

This example uses CASE with simple value matching to name product categories.

MySQL

SELECT
  product_name,
  CASE category_id
    WHEN 1 THEN 'Books'
    WHEN 2 THEN 'Electronics'
    ELSE 'Other'
  END AS category_name
FROM products;

This example groups employees by salary ranges.

MySQL

SELECT
  employee_name,
  salary,
  CASE
    WHEN salary < 3000 THEN 'Low'
    WHEN salary BETWEEN 3000 AND 7000 THEN 'Medium'
    ELSE 'High'
  END AS salary_level
FROM employees;

Sample Program

This creates a simple table of exam scores and uses CASE WHEN to mark each as Pass or Fail.

MySQL

CREATE TABLE exams (id INT, score INT);
INSERT INTO exams VALUES (1, 55), (2, 75), (3, 40);

SELECT id, score,
  CASE
    WHEN score >= 60 THEN 'Pass'
    ELSE 'Fail'
  END AS result
FROM exams;

Important Notes

CASE WHEN expressions can be used in SELECT, WHERE, ORDER BY, and other SQL clauses.

You can nest CASE expressions inside each other for more complex logic.

Always test your CASE logic with sample data to ensure it works as expected.

Summary

CASE WHEN lets you return different results based on conditions in your data.

It works like an IF-THEN-ELSE decision inside SQL queries.

Use it to create readable, flexible queries that handle multiple conditions.

Practice

1. What does the CASE WHEN expression do in a MySQL query?

easy

A. It creates a new table in the database.

B. It sorts the results in ascending order.

C. It deletes rows that meet a condition.

D. It returns different values based on specified conditions.

Solution

1. Step 1: Understand the purpose of CASE WHEN

   The CASE WHEN expression checks conditions and returns values accordingly.

2. Step 2: Compare with other options

   Creating tables, deleting rows, or sorting are different SQL operations, not done by CASE WHEN.

3. Final Answer:

   It returns different values based on specified conditions. -> Option D

4. Quick Check:

   CASE WHEN = conditional value return [OK]

Hint: CASE WHEN returns values based on conditions, like IF-THEN-ELSE [OK]

Common Mistakes:

• Thinking CASE WHEN creates or deletes tables
• Confusing CASE WHEN with ORDER BY sorting
• Assuming CASE WHEN changes data permanently

2. Which of the following is the correct syntax for a simple CASE WHEN expression in MySQL?

easy

A. SELECT CASE WHEN score > 50 THEN 'Pass' ELSE 'Fail' END FROM students;

B. SELECT CASE score > 50 THEN 'Pass' ELSE 'Fail' END FROM students;

C. SELECT CASE WHEN score > 50 THEN 'Pass' ELSE 'Fail' FROM students;

D. SELECT CASE score > 50 THEN 'Pass' ELSE 'Fail' END students;

Solution

1. Step 1: Check the correct CASE WHEN syntax

   The correct syntax includes CASE WHEN condition THEN result ELSE result END.

2. Step 2: Identify syntax errors in other options

   The other options miss keywords or punctuation: some omit WHEN, others forget END, or misplace FROM.

3. Final Answer:

   SELECT CASE WHEN score > 50 THEN 'Pass' ELSE 'Fail' END FROM students; -> Option A

4. Quick Check:

   Correct CASE WHEN syntax = SELECT CASE WHEN score > 50 THEN 'Pass' ELSE 'Fail' END FROM students; [OK]

Hint: CASE WHEN needs WHEN, THEN, ELSE, and END keywords [OK]

Common Mistakes:

• Omitting WHEN keyword
• Forgetting END at the end
• Misplacing FROM clause

3. Given the table employees with a column salary, what will this query return?

SELECT name, CASE WHEN salary >= 5000 THEN 'High' ELSE 'Low' END AS salary_level FROM employees;

medium

A. An error because CASE WHEN cannot be used in SELECT.

B. Only employees with salary greater than 5000 will be shown.

C. A list of employee names with 'High' or 'Low' based on salary threshold 5000.

D. A list of salaries labeled as 'High' or 'Low' without employee names.

Solution

1. Step 1: Understand the CASE WHEN usage in SELECT

   The query labels each employee's salary as 'High' if >= 5000, else 'Low'.

2. Step 2: Check what columns are selected

   It selects employee names and the computed salary_level, so all employees appear.

3. Final Answer:

   A list of employee names with 'High' or 'Low' based on salary threshold 5000. -> Option C

4. Quick Check:

   CASE WHEN labels salaries, all employees listed [OK]

Hint: CASE WHEN in SELECT returns labels per row without filtering [OK]

Common Mistakes:

• Thinking CASE WHEN filters rows instead of labeling
• Assuming query returns error
• Ignoring selected columns

4. Identify the error in this query:

SELECT name, CASE salary > 10000 THEN 'Rich' ELSE 'Average' END AS status FROM customers;

medium

A. Missing WHEN keyword after CASE.

B. Missing END keyword at the end.

C. Incorrect alias usage for CASE expression.

D. Using ELSE without THEN is invalid.

Solution

1. Step 1: Review CASE WHEN syntax

   CASE must be followed by WHEN before condition; here WHEN is missing.

2. Step 2: Check other parts of the query

   END keyword is present, alias is correct, ELSE is properly used after THEN.

3. Final Answer:

   Missing WHEN keyword after CASE. -> Option A

4. Quick Check:

   CASE requires WHEN after CASE [OK]

Hint: Always write CASE WHEN, not CASE condition directly [OK]

Common Mistakes:

• Omitting WHEN keyword
• Confusing ELSE and THEN order
• Forgetting END keyword

5. You want to categorize products by price into 'Cheap' (price < 20), 'Moderate' (20 ≤ price ≤ 100), and 'Expensive' (price > 100). Which CASE WHEN expression correctly implements this?

hard

A. CASE WHEN price < 20 THEN 'Cheap' WHEN price < 100 THEN 'Moderate' ELSE 'Expensive' END

B. CASE WHEN price < 20 THEN 'Cheap' WHEN price >= 20 AND price <= 100 THEN 'Moderate' ELSE 'Expensive' END

C. CASE WHEN price < 20 THEN 'Cheap' WHEN price > 20 AND price <= 100 THEN 'Moderate' ELSE 'Expensive' END

D. CASE WHEN price <= 20 THEN 'Cheap' WHEN price > 20 AND price < 100 THEN 'Moderate' ELSE 'Expensive' END

Solution

1. Step 1: Analyze the price ranges needed

   'Cheap' is price less than 20, 'Moderate' is price between 20 and 100 inclusive, 'Expensive' is above 100.

2. Step 2: Check each option's conditions

   CASE WHEN price < 20 THEN 'Cheap' WHEN price >= 20 AND price <= 100 THEN 'Moderate' ELSE 'Expensive' END correctly uses price < 20 for 'Cheap', price >= 20 and <= 100 for 'Moderate', else 'Expensive'. Others have incorrect boundaries.

3. Final Answer:

   CASE WHEN price < 20 THEN 'Cheap' WHEN price >= 20 AND price <= 100 THEN 'Moderate' ELSE 'Expensive' END -> Option B

4. Quick Check:

   Correct range checks = CASE WHEN price < 20 THEN 'Cheap' WHEN price >= 20 AND price <= 100 THEN 'Moderate' ELSE 'Expensive' END [OK]

Hint: Use inclusive ranges carefully with AND and >=, <= operators [OK]

Common Mistakes:

• Overlapping or missing ranges
• Using incorrect comparison operators
• Not covering all possible values