Simple CASE syntax in SQL - Time & Space Complexity

Time Complexity: Simple CASE syntax

O(n)

Understanding Time Complexity

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

Specifically, how does the number of rows affect the work done by the CASE expression?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

SELECT
  employee_id,
  salary,
  CASE department_id
    WHEN 10 THEN 'Accounting'
    WHEN 20 THEN 'Sales'
    ELSE 'Other'
  END AS department_name
FROM employees;

This query assigns a department name to each employee based on their department ID using a simple CASE expression.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The CASE expression is evaluated once for each row in the employees table.
• How many times: Exactly as many times as there are rows (employees).

How Execution Grows With Input

As the number of employees grows, the CASE expression runs once per employee.

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

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 CASE grows in a straight line with the number of rows.

Common Mistake

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

[OK] Correct: The CASE is evaluated for each row separately, so it repeats as many times as there are rows.

Interview Connect

Understanding how simple CASE expressions scale helps you explain query performance clearly and confidently.

Self-Check

"What if we added a nested CASE inside the simple CASE? How would the time complexity change?"

Practice

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

easy

A. It loops through rows and updates values conditionally.

B. It compares one expression to multiple values and returns a result for the first match.

C. It creates a new table based on conditions.

D. It deletes rows that match a condition.

Solution

1. Step 1: Understand the purpose of Simple CASE

   Simple CASE compares one expression against multiple possible values.

2. Step 2: Identify the result behavior

   It returns the result for the first matching value it finds.

3. Final Answer:

   It compares one expression to multiple values and returns a result for the first match. -> Option B

4. Quick Check:

   Simple CASE = compare expression to values [OK]

Hint: Simple CASE matches one value to many options [OK]

Common Mistakes:

• Thinking it loops through rows
• Confusing with IF statements
• Assuming it creates or deletes tables

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

easy

A. CASE expression WHEN value THEN result ELSE default END

B. IF expression THEN result ELSE default END

C. CASE WHEN expression THEN result END

D. SWITCH expression WHEN value THEN result END

Solution

1. Step 1: Recall Simple CASE syntax structure

   The correct syntax is: CASE expression WHEN value THEN result ELSE default END.

2. Step 2: Compare options

   CASE expression WHEN value THEN result ELSE default END matches this syntax exactly, others use incorrect keywords or structure.

3. Final Answer:

   CASE expression WHEN value THEN result ELSE default END -> Option A

4. Quick Check:

   Simple CASE syntax = CASE expr WHEN val THEN res ELSE def END [OK]

Hint: Simple CASE uses 'CASE expr WHEN val THEN res' format [OK]

Common Mistakes:

• Using WHEN without expression after CASE
• Confusing with IF or SWITCH syntax
• Omitting END keyword

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

SELECT EmployeeName,
 CASE DepartmentID
   WHEN 1 THEN 'HR'
   WHEN 2 THEN 'Sales'
   ELSE 'Other'
 END AS DeptName
FROM Employees;

medium

A. Employee names with department names 'HR', 'Sales', or 'Other' based on DepartmentID.

B. Only employees from HR and Sales departments.

C. An error because ELSE is missing.

D. Employee names with numeric DepartmentID values.

Solution

1. Step 1: Understand the CASE expression in the query

   The CASE checks DepartmentID: 1 returns 'HR', 2 returns 'Sales', others return 'Other'.

2. Step 2: Analyze the SELECT output

   The query returns EmployeeName and DeptName with the mapped department names or 'Other'.

3. Final Answer:

   Employee names with department names 'HR', 'Sales', or 'Other' based on DepartmentID. -> Option A

4. Quick Check:

   CASE maps DepartmentID to names [OK]

Hint: CASE returns first matching value or ELSE default [OK]

Common Mistakes:

• Thinking ELSE is required (it's optional)
• Assuming only HR and Sales appear
• Expecting numeric DepartmentID instead of names

4. Identify the error in this Simple CASE query:

SELECT ProductName,
 CASE ProductCategory
   WHEN 'A' THEN 'Category A'
   WHEN 'B' 'Category B'
   ELSE 'Other'
 END AS CategoryName
FROM Products;

medium

A. CASE expression should be inside parentheses.

B. Missing END keyword at the end.

C. Incorrect use of ELSE keyword.

D. Missing THEN keyword after WHEN 'B'.

Solution

1. Step 1: Review each WHEN clause syntax

   WHEN clauses must have THEN keyword before the result. The second WHEN is missing THEN.

2. Step 2: Check other parts of the query

   END keyword is present, ELSE is correctly used, parentheses are not required around CASE expression.

3. Final Answer:

   Missing THEN keyword after WHEN 'B'. -> Option D

4. Quick Check:

   WHEN must be followed by THEN [OK]

Hint: Each WHEN must be followed by THEN [OK]

Common Mistakes:

• Forgetting THEN after WHEN
• Thinking END is optional
• Adding unnecessary parentheses

5. You want to categorize sales amounts into 'Low', 'Medium', and 'High' using Simple CASE. Which query correctly assigns:
- 'Low' for 0
- 'Medium' for 1
- 'High' for 2 or more (default)?

SELECT SaleID,
 CASE SaleAmount
   WHEN 0 THEN 'Low'
   WHEN 1 THEN 'Medium'
   ELSE 'High'
 END AS SaleCategory
FROM Sales;

hard

A. Fails because ELSE cannot be used for multiple values.

B. Incorrect because Simple CASE cannot handle ranges like 2 or more.

C. Correctly categorizes sales as described.

D. Syntax error due to missing WHEN for 2.

Solution

1. Step 1: Understand Simple CASE limitations

   Simple CASE matches exact values; ELSE handles all other cases.

2. Step 2: Analyze the query logic

   WHEN 0 returns 'Low', WHEN 1 returns 'Medium', ELSE covers 2 or more as 'High'.

3. Final Answer:

   Correctly categorizes sales as described. -> Option C

4. Quick Check:

   ELSE covers all unmatched values [OK]

Hint: Use ELSE for all other values in Simple CASE [OK]

Common Mistakes:

• Thinking ELSE can't cover multiple values
• Trying to use ranges in Simple CASE
• Expecting WHEN for every possible value