CASE WHEN expression in MySQL - Time & Space Complexity

Time Complexity: CASE WHEN expression

O(n)

Understanding Time Complexity

We want to understand how the time it takes to run a SQL query with a CASE WHEN expression changes as the data grows.

Specifically, we ask: How does the number of rows affect the work done by CASE WHEN?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

SELECT
  id,
  name,
  CASE
    WHEN score >= 90 THEN 'A'
    WHEN score >= 80 THEN 'B'
    WHEN score >= 70 THEN 'C'
    ELSE 'F'
  END AS grade
FROM students;

This query assigns a grade to each student based on their score using CASE WHEN.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The CASE WHEN expression is evaluated once for each row in the students table.
• How many times: Exactly once per row, so as many times as there are rows.

How Execution Grows With Input

As the number of rows grows, the database checks the CASE WHEN conditions for each row separately.

Input Size (n)	Approx. Operations
10	10 CASE checks
100	100 CASE checks
1000	1000 CASE checks

Pattern observation: The work grows directly with the number of rows; doubling rows doubles the work.

Final Time Complexity

Time Complexity: O(n)

This means the time to run the query grows linearly with the number of rows in the table.

Common Mistake

[X] Wrong: "CASE WHEN runs only once regardless of rows because it's just a condition."

[OK] Correct: The CASE WHEN expression is checked for every row, so it runs as many times as there are rows.

Interview Connect

Understanding how CASE WHEN scales helps you explain query performance clearly and shows you know how SQL processes data row by row.

Self-Check

"What if we added a JOIN that doubles the number of rows? How would the time complexity change?"

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