Searched CASE syntax in SQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of Searched CASE with multiple conditions

Given the table Employees with columns id, salary, and department, what is the output of the following query?

SELECT id, salary,
 CASE
   WHEN salary > 70000 THEN 'High'
   WHEN salary >= 40000 AND salary <= 70000 THEN 'Medium'
   ELSE 'Low'
 END AS SalaryLevel
FROM Employees
WHERE department = 'Sales';

Assume the Employees table has these rows:

id | salary | department
1  | 80000  | Sales
2  | 50000  | Sales
3  | 30000  | Sales
4  | 60000  | HR

SQL

SELECT id, salary,
 CASE
   WHEN salary > 70000 THEN 'High'
   WHEN salary >= 40000 AND salary <= 70000 THEN 'Medium'
   ELSE 'Low'
 END AS SalaryLevel
FROM Employees
WHERE department = 'Sales';

A [{"id":1,"salary":80000,"SalaryLevel":"High"},{"id":2,"salary":50000,"SalaryLevel":"Low"},{"id":3,"salary":30000,"SalaryLevel":"Low"}]

B [{"id":1,"salary":80000,"SalaryLevel":"High"},{"id":2,"salary":50000,"SalaryLevel":"Medium"},{"id":3,"salary":30000,"SalaryLevel":"Low"}]

C [{"id":1,"salary":80000,"SalaryLevel":"Medium"},{"id":2,"salary":50000,"SalaryLevel":"Medium"},{"id":3,"salary":30000,"SalaryLevel":"Low"}]

D [{"id":1,"salary":80000,"SalaryLevel":"High"},{"id":2,"salary":50000,"SalaryLevel":"Medium"}]

💡 Hint

Remember the searched CASE evaluates conditions in order and stops at the first true condition.

The CASE expression checks salary > 70000 first, so id=1 gets 'High'. Then salary between 40000 and 70000 is 'Medium' for id=2. The ELSE covers salaries below 40000, so id=3 gets 'Low'. Only employees in Sales department are included.

🧠 Conceptual

intermediate

Understanding evaluation order in Searched CASE

In a searched CASE expression, why is the order of WHEN conditions important?

Consider:

CASE
  WHEN score >= 90 THEN 'A'
  WHEN score >= 80 THEN 'B'
  WHEN score >= 70 THEN 'C'
  ELSE 'F'
END

What happens if the order is changed to check score >= 70 first?

A The first matching condition is used, so if <code>score &gt;= 70</code> is first, all scores 70 and above get 'C', ignoring higher grades.

B The CASE expression evaluates all conditions and returns the highest grade matching the score.

C The CASE expression will cause a syntax error if conditions are reordered.

D The order does not matter; all conditions are checked and the best match is returned.

💡 Hint

Think about how CASE stops checking conditions once one is true.

Searched CASE evaluates conditions top to bottom and returns the result of the first true condition. If a broader condition like score >= 70 is first, it matches many scores and later conditions are ignored.

📝 Syntax

advanced

Identify the syntax error in this searched CASE query

Which option contains a syntax error in the searched CASE expression?

SELECT id,
 CASE
   WHEN age > 60 THEN 'Senior'
   WHEN age >= 18 THEN 'Adult'
   ELSE 'Minor'
 END AS AgeGroup
FROM Users;

A CASE WHEN age > 60 THEN 'Senior' WHEN age >= 18 THEN 'Adult' ELSE 'Minor' END

B CASE WHEN age > 60 THEN 'Senior' WHEN age >= 18 THEN 'Adult' ELSE 'Minor' END AS AgeGroup

C DNE 'roniM' ESLE 'tludA' NEHT 81 => ega NEHW 'roineS' NEHT 06 > ega NEHW ESAC

D CASE age WHEN > 60 THEN 'Senior' WHEN >= 18 THEN 'Adult' ELSE 'Minor' END

💡 Hint

Check the syntax for searched CASE vs simple CASE.

Option D incorrectly uses searched CASE syntax with a simple CASE style (CASE age WHEN > 60 ...), which is invalid. The searched CASE requires WHEN conditions with full boolean expressions.

❓ optimization

advanced

Optimizing searched CASE for performance

You have a large table Orders with a total_amount column. You want to classify orders as 'Small', 'Medium', or 'Large' using searched CASE:

CASE
  WHEN total_amount < 100 THEN 'Small'
  WHEN total_amount < 500 THEN 'Medium'
  ELSE 'Large'
END

Which approach can improve query performance when filtering by these categories?

A Rewrite the CASE as multiple OR conditions in the WHERE clause without indexing.

B Use nested CASE expressions inside the query to reduce evaluation time.

C Create a computed column with the CASE expression and index it, then filter on that column.

D Remove the CASE expression and filter only by total_amount ranges in the WHERE clause.

💡 Hint

Think about indexing and how it helps filtering.

Creating a computed column with the CASE expression and indexing it allows the database to quickly filter by category without recalculating CASE for every row.

🔧 Debug

expert

Debugging unexpected output from searched CASE

A developer writes this query:

SELECT product_id,
 CASE
   WHEN stock < 10 THEN 'Low'
   WHEN stock < 50 THEN 'Medium'
   WHEN stock < 100 THEN 'High'
   ELSE 'Very High'
 END AS StockLevel
FROM Inventory;

They notice that products with stock 5 are labeled 'Low' as expected, but products with stock 30 are labeled 'Low' instead of 'Medium'. What is the most likely cause?

A The stock column is stored as text, so comparisons behave unexpectedly.

B The CASE conditions are evaluated in order, so stock < 10 matches first, but 30 is not less than 10, so it should be 'Medium'.

C There is a hidden trigger changing stock values before query execution.

D The query is missing parentheses around conditions causing wrong evaluation.

💡 Hint

Check data types and how comparisons work.

If stock is stored as text, '30' < '10' can be false in string comparison because '3' is greater than '1' in ASCII order, causing wrong CASE branch to match.