Nested CASE expressions in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to return 'High' when score is above 80 using a CASE expression.

SQL

SELECT CASE WHEN score [1] 80 THEN 'High' ELSE 'Low' END AS performance FROM results;

Drag options to blanks, or click blank then click option'

A=

B<

C<=

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>' will label scores below 80 as 'High'.

Using '=' will only label scores exactly 80 as 'High'.

The condition checks if the score is greater than 80 to label it as 'High'.

2fill in blank

medium

Complete the code to return 'Medium' when score is between 50 and 80 using nested CASE expressions.

SQL

SELECT CASE WHEN score > 80 THEN 'High' WHEN score [1] 50 THEN 'Medium' ELSE 'Low' END AS performance FROM results;

Drag options to blanks, or click blank then click option'

A<

B>=

C=

D<=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<=' instead of '>=' will incorrectly classify scores below 50 as 'Medium'.

Using '=' only matches score exactly 50.

The condition checks if the score is greater than or equal to 50 to assign 'Medium' for scores between 50 and 80.

3fill in blank

hard

Fix the error in the nested CASE expression to correctly classify scores.

SQL

SELECT CASE WHEN score > 80 THEN 'High' WHEN score [1] 50 THEN 'Medium' ELSE 'Low' END AS performance FROM results;

Drag options to blanks, or click blank then click option'

A>=

B<=

C<

D=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<=' will incorrectly classify scores below 50 as 'Medium'.

Using '=' only matches score exactly 50.

The condition should check if score is greater than or equal to 50 to classify as 'Medium'.

4fill in blank

hard

Fill both blanks to classify scores into 'High', 'Medium', or 'Low' using nested CASE expressions.

SQL

SELECT CASE WHEN score [1] 80 THEN 'High' WHEN score [2] 50 THEN 'Medium' ELSE 'Low' END AS performance FROM results;

Drag options to blanks, or click blank then click option'

A>

B<=

C>=

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the operators will misclassify scores.

Using '<=' in the first blank will cause wrong results.

The first condition checks if score is greater than 80 for 'High'. The second checks if score is greater than or equal to 50 for 'Medium'.

5fill in blank

hard

Fill all three blanks to create a nested CASE expression that classifies scores into 'Excellent', 'Good', 'Average', or 'Poor'.

SQL

SELECT CASE WHEN score [1] 90 THEN 'Excellent' WHEN score [2] 75 THEN 'Good' WHEN score [3] 50 THEN 'Average' ELSE 'Poor' END AS performance FROM results;

Drag options to blanks, or click blank then click option'

A>=

B<

C>

D<=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>' for the first two blanks will misclassify scores.

Using '<=' instead of '>=' for the last blank will misclassify low scores.

The first two blanks use '>' to check if score is above 90 and 75. The third uses '>=' to check greater than or equal to 50 for 'Average'.

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