Nested CASE expressions in SQL

Introduction

Nested CASE expressions help you make decisions inside other decisions in your data queries. This lets you check multiple conditions step by step.

You want to assign categories based on multiple levels of conditions.

You need to check one condition, and inside it check another condition.

You want to create detailed labels or groups from your data.

You want to replace complex IF-ELSE logic in your query with readable SQL.

You want to handle special cases differently inside a general case.

Syntax

SQL

CASE
  WHEN condition1 THEN
    CASE
      WHEN condition2 THEN result2
      ELSE result3
    END
  ELSE result1
END

You can put one CASE expression inside another CASE expression.

Each CASE must end with END to close it.

Examples

This example assigns grades. If score is 90 or more, grade is A. If score is between 80 and 89, it checks again: 85 or more is B+, else B. Otherwise, grade is C.

SQL

SELECT
  CASE
    WHEN score >= 90 THEN 'A'
    WHEN score >= 80 THEN
      CASE
        WHEN score >= 85 THEN 'B+'
        ELSE 'B'
      END
    ELSE 'C'
  END AS grade
FROM students;

Here, products in the 'Fruit' category get a price label based on price. Others get 'Other'.

SQL

SELECT
  product,
  CASE
    WHEN category = 'Fruit' THEN
      CASE
        WHEN price < 1 THEN 'Cheap Fruit'
        ELSE 'Expensive Fruit'
      END
    ELSE 'Other'
  END AS price_label
FROM products;

Sample Program

This query labels sales by region and amount. It first checks the region, then inside that, checks if the amount is high or low.

SQL

WITH sales AS (
  SELECT 'North' AS region, 120 AS amount UNION ALL
  SELECT 'North', 80 UNION ALL
  SELECT 'South', 50 UNION ALL
  SELECT 'South', 200
)
SELECT
  region,
  amount,
  CASE
    WHEN region = 'North' THEN
      CASE
        WHEN amount > 100 THEN 'High North Sale'
        ELSE 'Low North Sale'
      END
    WHEN region = 'South' THEN
      CASE
        WHEN amount > 100 THEN 'High South Sale'
        ELSE 'Low South Sale'
      END
    ELSE 'Other Region'
  END AS sale_category
FROM sales;

Important Notes

Always close each CASE with END to avoid errors.

Nested CASE expressions can make queries longer but help organize complex logic clearly.

Indent your CASE statements to keep the code easy to read.

Summary

Nested CASE expressions let you check conditions inside other conditions.

They help create detailed categories or labels in your data.

Remember to close each CASE with END and keep your code readable.

Practice

1. What is the main purpose of using nested CASE expressions in SQL?

easy

A. To check multiple conditions inside another condition for detailed results

B. To create new tables from existing data

C. To delete rows based on conditions

D. To join two tables together

Solution

1. Step 1: Understand the role of CASE expressions

   CASE expressions allow conditional logic to return different values based on conditions.

2. Step 2: Recognize nested CASE usage

   Nested CASE means putting one CASE inside another to check conditions within conditions for more detailed categorization.

3. Final Answer:

   To check multiple conditions inside another condition for detailed results -> Option A

4. Quick Check:

   Nested CASE = detailed condition checks [OK]

Hint: Nested CASE means CASE inside CASE for detailed checks [OK]

Common Mistakes:

• Confusing CASE with JOIN operations
• Thinking CASE creates or deletes tables
• Ignoring the need to close each CASE with END

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

easy

A. CASE WHEN condition1 THEN IF condition2 THEN result1 ELSE result2 END ELSE result3 END

B. CASE WHEN condition1 THEN CASE condition2 THEN result1 ELSE result2 END ELSE result3 END

C. IF condition1 THEN CASE WHEN condition2 THEN result1 ELSE result2 END ELSE result3 END

D. CASE WHEN condition1 THEN CASE WHEN condition2 THEN result1 ELSE result2 END ELSE result3 END

Solution

1. Step 1: Review correct CASE syntax

   CASE expressions use WHEN ... THEN ... ELSE ... END structure; nested CASE must follow this pattern inside THEN.

2. Step 2: Identify correct nested CASE

   CASE WHEN condition1 THEN CASE WHEN condition2 THEN result1 ELSE result2 END ELSE result3 END correctly nests a CASE inside THEN with proper WHEN, THEN, ELSE, and END keywords.

3. Final Answer:

   CASE WHEN condition1 THEN CASE WHEN condition2 THEN result1 ELSE result2 END ELSE result3 END -> Option D

4. Quick Check:

   Nested CASE syntax = CASE WHEN condition1 THEN CASE WHEN condition2 THEN result1 ELSE result2 END ELSE result3 END [OK]

Hint: Nested CASE uses WHEN-THEN inside THEN with END for each CASE [OK]

Common Mistakes:

• Using IF instead of CASE inside SQL
• Missing END keyword for inner CASE
• Incorrect keyword order in nested CASE

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

SELECT Salary, CASE WHEN Salary > 70000 THEN CASE WHEN Salary > 90000 THEN 'High' ELSE 'Medium' END ELSE 'Low' END AS SalaryLevel FROM Employees;

medium

A. Labels all salaries above 70000 as 'High' and others as 'Low'

B. Labels salaries above 90000 as 'High', between 70001 and 90000 as 'Medium', others as 'Low'

C. Labels all salaries below 70000 as 'Medium' and others as 'High'

D. Returns an error due to incorrect CASE syntax

Solution

1. Step 1: Analyze outer CASE condition

   If Salary > 70000, then check inner CASE; else label 'Low'.

2. Step 2: Analyze inner CASE condition

   Inside outer WHEN true, if Salary > 90000 then 'High', else 'Medium'.

3. Final Answer:

   Labels salaries above 90000 as 'High', between 70001 and 90000 as 'Medium', others as 'Low' -> Option B

4. Quick Check:

   Nested CASE categorizes salary levels correctly [OK]

Hint: Check outer condition first, then inner CASE for detailed labels [OK]

Common Mistakes:

• Ignoring inner CASE and labeling all above 70000 as same
• Confusing syntax causing errors
• Misreading comparison operators

4. Identify the error in this nested CASE SQL snippet:

SELECT CASE WHEN score > 80 THEN CASE score > 90 THEN 'Excellent' ELSE 'Good' END ELSE 'Average' END AS Grade FROM Results;

medium

A. Using wrong comparison operator

B. Missing END keyword for outer CASE

C. Missing WHEN keyword before inner condition

D. Incorrect ELSE placement inside inner CASE

Solution

1. Step 1: Check inner CASE syntax

   Inner CASE lacks WHEN before condition; it should be 'CASE WHEN score > 90 THEN ...'

2. Step 2: Verify overall CASE structure

   Outer CASE has proper WHEN and END; error is only in inner CASE missing WHEN keyword.

3. Final Answer:

   Missing WHEN keyword before inner condition -> Option C

4. Quick Check:

   Inner CASE needs WHEN before condition [OK]

Hint: Every CASE condition must start with WHEN [OK]

Common Mistakes:

• Forgetting WHEN in inner CASE
• Assuming END is missing when it is present
• Misplacing ELSE inside CASE

5. You want to classify products based on Price and Stock using nested CASE. If Price > 100 and Stock > 50, label 'Premium-Available'. If Price > 100 and Stock ≤ 50, label 'Premium-Limited'. Otherwise, label 'Standard'. Which SQL snippet correctly implements this?

hard

A. CASE WHEN Price > 100 THEN CASE WHEN Stock > 50 THEN 'Premium-Available' ELSE 'Premium-Limited' END ELSE 'Standard' END

B. CASE WHEN Price > 100 AND Stock > 50 THEN 'Premium-Available' WHEN Price > 100 AND Stock <= 50 THEN 'Premium-Limited' ELSE 'Standard' END

C. CASE WHEN Price > 100 THEN 'Premium' ELSE CASE WHEN Stock > 50 THEN 'Available' ELSE 'Limited' END END

D. CASE WHEN Price > 100 THEN CASE Stock > 50 THEN 'Premium-Available' ELSE 'Premium-Limited' END ELSE 'Standard' END

Solution

1. Step 1: Understand the nested conditions

   First check if Price > 100, then inside check Stock > 50 or not.

2. Step 2: Match correct syntax and logic

   CASE WHEN Price > 100 THEN CASE WHEN Stock > 50 THEN 'Premium-Available' ELSE 'Premium-Limited' END ELSE 'Standard' END correctly nests CASE inside THEN with proper WHEN, THEN, ELSE, and END keywords matching the problem.

3. Final Answer:

   CASE WHEN Price > 100 THEN CASE WHEN Stock > 50 THEN 'Premium-Available' ELSE 'Premium-Limited' END ELSE 'Standard' END -> Option A

4. Quick Check:

   Nested CASE matches Price then Stock checks [OK]

Hint: Check outer condition first, nest inner CASE for second condition [OK]

Common Mistakes:

• Using AND instead of nested CASE
• Missing WHEN in inner CASE
• Incorrect ELSE placement