Searched CASE syntax in SQL - Time & Space Complexity

Time Complexity: Searched CASE syntax

O(n)

Understanding Time Complexity

We want to understand how the time it takes to run a searched CASE statement changes as the data grows.

How does the number of rows affect the work done inside the CASE?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

SELECT
  employee_id,
  salary,
  CASE
    WHEN salary < 30000 THEN 'Low'
    WHEN salary < 70000 THEN 'Medium'
    ELSE 'High'
  END AS salary_level
FROM employees;

This code classifies each employee's salary into categories using searched CASE conditions.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The CASE conditions are checked once for each row in the employees table.
• How many times: Once per row, so as many times as there are rows.

How Execution Grows With Input

Each row requires checking the CASE conditions in order, so the total work grows with the number of rows.

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

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

Final Time Complexity

Time Complexity: O(n)

This means the time to run the query grows in a straight line with the number of rows.

Common Mistake

[X] Wrong: "The CASE statement runs only once regardless of rows."

[OK] Correct: The CASE is evaluated for every row, so the total work depends on how many rows there are.

Interview Connect

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

Self-Check

"What if we added more WHEN conditions inside the CASE? How would the time complexity change?"

Practice

1. What does the searched CASE expression in SQL do?

easy

A. Deletes rows from a table

B. Joins two tables together

C. Creates a new table

D. Returns different results based on multiple conditions

Solution

1. Step 1: Understand the purpose of searched CASE

   The searched CASE expression evaluates multiple conditions and returns a result for the first true condition.

2. Step 2: Compare with other SQL operations

   Unlike joins, deletes, or table creation, CASE is used to return values conditionally within queries.

3. Final Answer:

   Returns different results based on multiple conditions -> Option D

4. Quick Check:

   Searched CASE = conditional results [OK]

Hint: Think of CASE as SQL's if-else for conditions [OK]

Common Mistakes:

• Confusing CASE with JOIN operations
• Thinking CASE modifies tables
• Assuming CASE deletes data

2. Which of the following is the correct syntax for a searched CASE expression in SQL?

easy

A. CASE WHEN condition THEN result ELSE default END

B. CASE condition WHEN THEN result ELSE END

C. CASE THEN condition WHEN result ELSE END

D. CASE WHEN THEN condition ELSE result END

Solution

1. Step 1: Recall the searched CASE syntax

   The correct syntax starts with CASE, followed by WHEN condition THEN result, optional ELSE, and ends with END.

2. Step 2: Check each option

   Only CASE WHEN condition THEN result ELSE default END follows the correct order: CASE WHEN condition THEN result ELSE default END.

3. Final Answer:

   CASE WHEN condition THEN result ELSE default END -> Option A

4. Quick Check:

   Syntax = CASE WHEN ... THEN ... ELSE ... END [OK]

Hint: Remember: WHEN condition THEN result, ELSE optional, END last [OK]

Common Mistakes:

• Mixing order of WHEN and THEN
• Omitting END keyword
• Placing ELSE before THEN

3. Given the table Employees with column Salary, what will this query return?

SELECT Salary, CASE WHEN Salary > 5000 THEN 'High' WHEN Salary > 3000 THEN 'Medium' ELSE 'Low' END AS SalaryLevel FROM Employees;

medium

A. Returns only salaries greater than 5000

B. Shows an error because ELSE is missing

C. Labels each salary as 'High', 'Medium', or 'Low' based on value

D. Labels all salaries as 'Low'

Solution

1. Step 1: Analyze the CASE conditions

   The CASE checks if Salary > 5000, then returns 'High'; else if Salary > 3000, returns 'Medium'; otherwise 'Low'.

2. Step 2: Understand the query output

   Each row shows Salary and its label based on these conditions, so salaries are categorized correctly.

3. Final Answer:

   Labels each salary as 'High', 'Medium', or 'Low' based on value -> Option C

4. Quick Check:

   CASE categorizes salaries correctly [OK]

Hint: Read CASE conditions top to bottom, first true wins [OK]

Common Mistakes:

• Thinking ELSE is required and missing causes error
• Assuming query filters rows instead of labeling
• Ignoring order of WHEN conditions

4. Identify the error in this searched CASE expression:

SELECT Name, CASE WHEN Age > 18 THEN 'Adult' WHEN Age < 18 THEN 'Minor' END AS AgeGroup FROM People;

medium

A. Incorrect use of THEN keyword

B. No error, query is correct

C. No END keyword to close CASE

D. Missing ELSE clause for other cases

Solution

1. Step 1: Check CASE syntax completeness

   The CASE has WHEN conditions, THEN results, and ends with END keyword properly.

2. Step 2: Consider ELSE clause necessity

   ELSE is optional; if no ELSE, NULL is returned for unmatched cases, which is valid.

3. Final Answer:

   No error, query is correct -> Option B

4. Quick Check:

   ELSE optional, END required, syntax correct [OK]

Hint: ELSE is optional; END is mandatory [OK]

Common Mistakes:

• Assuming ELSE is always required
• Forgetting END keyword
• Misplacing THEN keyword

5. You want to classify products by price range using searched CASE. Which query correctly assigns 'Cheap' for prices below 50, 'Affordable' for prices between 50 and 150, and 'Expensive' for prices above 150?

hard

A. SELECT ProductName, CASE WHEN Price < 50 THEN 'Cheap' WHEN Price BETWEEN 50 AND 150 THEN 'Affordable' ELSE 'Expensive' END AS PriceRange FROM Products;

B. SELECT ProductName, CASE WHEN Price > 150 THEN 'Cheap' WHEN Price BETWEEN 50 AND 150 THEN 'Affordable' ELSE 'Expensive' END AS PriceRange FROM Products;

C. SELECT ProductName, CASE WHEN Price < 50 THEN 'Cheap' WHEN Price > 150 THEN 'Affordable' ELSE 'Expensive' END AS PriceRange FROM Products;

D. SELECT ProductName, CASE WHEN Price BETWEEN 50 AND 150 THEN 'Cheap' WHEN Price < 50 THEN 'Affordable' ELSE 'Expensive' END AS PriceRange FROM Products;

Solution

1. Step 1: Match conditions to price ranges

   Price < 50 should be 'Cheap', 50 to 150 'Affordable', above 150 'Expensive'.

2. Step 2: Verify each option's logic

   SELECT ProductName, CASE WHEN Price < 50 THEN 'Cheap' WHEN Price BETWEEN 50 AND 150 THEN 'Affordable' ELSE 'Expensive' END AS PriceRange FROM Products; correctly orders conditions and labels prices as required. Others mislabel or misorder conditions.

3. Final Answer:

   SELECT ProductName, CASE WHEN Price < 50 THEN 'Cheap' WHEN Price BETWEEN 50 AND 150 THEN 'Affordable' ELSE 'Expensive' END AS PriceRange FROM Products; -> Option A

4. Quick Check:

   Conditions match price ranges correctly [OK]

Hint: Order CASE conditions from smallest to largest range [OK]

Common Mistakes:

• Mixing condition order causing wrong labels
• Using wrong comparison operators
• Misplacing BETWEEN ranges