Why CASE expressions are needed in SQL - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to return 'Adult' if age is 18 or more, else 'Minor'.

SQL

SELECT name, CASE WHEN age >= [1] THEN 'Adult' ELSE 'Minor' END AS status FROM people;

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

A18

B21

C16

D20

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong age value like 21 or 16 changes the classification incorrectly.

Forgetting to use '>=' causes wrong results.

The CASE expression checks if age is 18 or more to classify as 'Adult'.

2fill in blank

medium

Complete the code to assign 'Pass' if score is 50 or more, otherwise 'Fail'.

SQL

SELECT student, CASE WHEN score >= [1] THEN 'Pass' ELSE 'Fail' END AS result FROM exams;

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

A40

B50

C60

D70

Attempts:

3 left

💡 Hint

Common Mistakes

Using 40 or 60 changes pass/fail results incorrectly.

Not using '>=' causes wrong classification.

The passing score is 50, so the CASE expression uses 50 to decide.

3fill in blank

hard

Fix the error in the CASE expression to categorize salary as 'High' if above 70000, else 'Low'.

SQL

SELECT employee, CASE WHEN salary [1] 70000 THEN 'High' ELSE 'Low' END AS category FROM staff;

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

A<=

B<

C>=

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' or '<=' reverses the logic.

Using '>=' includes 70000 as 'High' which may not be intended.

To classify salary as 'High' when it is above 70000, use '>' operator.

4fill in blank

hard

Fill both blanks to classify temperature as 'Cold' if below 15, 'Warm' if between 15 and 25, else 'Hot'.

SQL

SELECT city, CASE WHEN temp < [1] THEN 'Cold' WHEN temp <= [2] THEN 'Warm' ELSE 'Hot' END AS weather FROM forecast;

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

A15

B20

C25

D30

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing thresholds causes wrong weather classification.

Using wrong comparison operators changes results.

Temperature below 15 is 'Cold', up to 25 is 'Warm', else 'Hot'.

5fill in blank

hard

Fill all three blanks to assign grade: 'A' if score >= 90, 'B' if >= 75, else 'C'.

SQL

SELECT student, CASE WHEN score >= [1] THEN 'A' WHEN score >= [2] THEN 'B' ELSE [3] END AS grade FROM results;

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

A90

B75

C'C'

D'B'

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong thresholds mixes grades.

Not quoting grade letters causes syntax errors.

Grades are assigned based on score thresholds: 90 for A, 75 for B, else C.

Practice

1. Why do we need CASE expressions in SQL queries?

easy

A. To change the database schema

B. To create new tables in the database

C. To delete rows from a table

D. To return different values based on conditions within a query

Solution

1. Step 1: Understand the purpose of CASE expressions

   CASE expressions allow SQL queries to return different results depending on conditions you specify.

2. Step 2: Compare with other SQL operations

   Creating tables, deleting rows, or changing schema are done by other SQL commands, not CASE.

3. Final Answer:

   To return different values based on conditions within a query -> Option D

4. Quick Check:

   CASE = conditional output [OK]

Hint: CASE is for conditional results inside queries [OK]

Common Mistakes:

• Thinking CASE creates or deletes tables
• Confusing CASE with schema changes
• Assuming CASE modifies data directly

2. Which of the following is the correct syntax to start a CASE expression in SQL?

easy

A. IF condition THEN result ELSE other END

B. CASE WHEN condition THEN result ELSE other END

C. SWITCH condition THEN result ELSE other END

D. SELECT CASE condition THEN result ELSE other

Solution

1. Step 1: Recall the standard CASE syntax

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

2. Step 2: Identify incorrect options

   IF and SWITCH are not valid SQL CASE keywords; SELECT CASE is incomplete syntax.

3. Final Answer:

   CASE WHEN condition THEN result ELSE other END -> Option B

4. Quick Check:

   CASE syntax = WHEN...THEN...END [OK]

Hint: CASE always uses WHEN...THEN...END structure [OK]

Common Mistakes:

• Using IF instead of CASE in SQL
• Omitting END keyword
• Starting CASE with SELECT

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

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

medium

A. Only salaries below 5000 with label 'Low'

B. Only salaries above 5000 with label 'High'

C. A list of salaries with labels 'High' for salaries above 5000 and 'Low' otherwise

D. An error because CASE cannot be used in SELECT

Solution

1. Step 1: Understand the CASE logic in SELECT

   The CASE checks each Salary; if greater than 5000, label 'High', else 'Low'.

2. Step 2: Check what rows are returned

   The query returns all rows with Salary and corresponding label, no rows are filtered out.

3. Final Answer:

   A list of salaries with labels 'High' for salaries above 5000 and 'Low' otherwise -> Option C

4. Quick Check:

   CASE labels salaries conditionally [OK]

Hint: CASE in SELECT labels rows, does not filter [OK]

Common Mistakes:

• Thinking CASE filters rows like WHERE
• Assuming only 'High' salaries show
• Believing CASE causes syntax error in SELECT

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

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

medium

A. Missing WHEN keyword after CASE

B. Missing END keyword at the end

C. Incorrect alias usage for SalaryLevel

D. CASE cannot be used in SELECT clause

Solution

1. Step 1: Review CASE syntax

   CASE must be followed by WHEN condition THEN result; here WHEN is missing.

2. Step 2: Check other parts

   END keyword is present; alias usage is correct; CASE is valid in SELECT.

3. Final Answer:

   Missing WHEN keyword after CASE -> Option A

4. Quick Check:

   CASE requires WHEN after CASE [OK]

Hint: CASE always needs WHEN after it [OK]

Common Mistakes:

• Omitting WHEN keyword
• Confusing END with alias
• Thinking CASE can't be in SELECT

5. You want to assign a grade based on Score in a table:
- Score ≥ 90: 'A'
- Score ≥ 80 and < 90: 'B'
- Score ≥ 70 and < 80: 'C'
- Otherwise: 'F'
Which CASE expression correctly implements this grading logic?

hard

A. CASE WHEN Score ≥ 90 THEN 'A' WHEN Score ≥ 80 THEN 'B' WHEN Score ≥ 70 THEN 'C' ELSE 'F' END

B. CASE WHEN Score < 70 THEN 'F' WHEN Score < 90 THEN 'C' WHEN Score < 80 THEN 'B' ELSE 'A' END

C. CASE WHEN Score ≥ 70 THEN 'C' WHEN Score ≥ 80 THEN 'B' WHEN Score ≥ 90 THEN 'A' ELSE 'F' END

D. CASE Score WHEN ≥ 90 THEN 'A' WHEN ≥ 80 THEN 'B' WHEN ≥ 70 THEN 'C' ELSE 'F' END

Solution

1. Step 1: Understand the grading order

   Grades must be checked from highest to lowest score to assign correct grade.

2. Step 2: Analyze each option

   CASE WHEN Score ≥ 90 THEN 'A' WHEN Score ≥ 80 THEN 'B' WHEN Score ≥ 70 THEN 'C' ELSE 'F' END checks from highest to lowest correctly; CASE WHEN Score < 70 THEN 'F' WHEN Score < 90 THEN 'C' WHEN Score < 80 THEN 'B' ELSE 'A' END incorrectly assigns scores 70-89 to 'C'; CASE WHEN Score ≥ 70 THEN 'C' WHEN Score ≥ 80 THEN 'B' WHEN Score ≥ 90 THEN 'A' ELSE 'F' END checks in wrong order; CASE Score WHEN ≥ 90 THEN 'A' WHEN ≥ 80 THEN 'B' WHEN ≥ 70 THEN 'C' ELSE 'F' END uses invalid CASE syntax.

3. Final Answer:

   CASE WHEN Score ≥ 90 THEN 'A' WHEN Score ≥ 80 THEN 'B' WHEN Score ≥ 70 THEN 'C' ELSE 'F' END -> Option A

4. Quick Check:

   Order conditions high to low in CASE [OK]

Hint: Check conditions from highest to lowest in CASE [OK]

Common Mistakes:

• Checking conditions in wrong order
• Using CASE with invalid syntax
• Assuming CASE compares values directly without WHEN