CASE in WHERE clause in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to filter rows where the status is 'active'.

SQL

SELECT * FROM users WHERE status = [1];

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

A'inactive'

B'active'

Cactive

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to use quotes around string literals.

Using a number instead of a string.

The status column is a string, so the value 'active' must be enclosed in single quotes in SQL.

2fill in blank

medium

Complete the code to use CASE in WHERE clause to filter users by role.

SQL

SELECT * FROM users WHERE role = CASE WHEN is_admin = 1 THEN [1] ELSE 'user' END;

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

A'admin'

Badmin

C'guest'

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Not using quotes around 'admin'.

Using a number instead of a string.

The CASE expression returns a string, so 'admin' must be in quotes.

3fill in blank

hard

Fix the error in the CASE expression inside the WHERE clause.

SQL

SELECT * FROM orders WHERE CASE [1] WHEN 'shipped' THEN 1 ELSE 0 END = 1;

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

Astatus

Border_date

C1

D'shipped'

Attempts:

3 left

💡 Hint

Common Mistakes

Using a literal instead of a column in CASE.

Putting quotes around the column name.

The CASE expression compares the column 'status' to the WHEN value.

4fill in blank

hard

Fill both blanks to filter products by price range using CASE in WHERE clause.

SQL

SELECT * FROM products WHERE price [1] CASE WHEN category = 'luxury' THEN 1000 ELSE [2] END;

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

A>

B<

C500

D100

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong comparison operators.

Confusing the price values for categories.

The query filters products with price greater than 1000 if category is 'luxury', otherwise greater than 500.

5fill in blank

hard

Fill all three blanks to filter employees by department and salary using CASE in WHERE clause.

SQL

SELECT * FROM employees WHERE salary [1] CASE WHEN department = [2] THEN [3] ELSE 50000 END;

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

A'HR'

B>

C60000

D'Sales'

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting quotes around department names.

Using wrong comparison operator for salary.

The query selects employees in HR with salary > 60000, or others with salary > 50000.

Practice

1. What does using CASE in a WHERE clause allow you to do in SQL?

easy

A. Sort the results in multiple ways

B. Create new columns in the result set

C. Apply different filter conditions based on row values

D. Join tables conditionally

Solution

1. Step 1: Understand the role of CASE in WHERE

   The CASE expression in WHERE lets you choose which condition to apply depending on data values in each row.

2. Step 2: Compare with other options

   Creating new columns is done with SELECT, sorting with ORDER BY, and joining with JOIN, not CASE in WHERE.

3. Final Answer:

   Apply different filter conditions based on row values -> Option C

4. Quick Check:

   CASE in WHERE = conditional filtering [OK]

Hint: CASE in WHERE picks filters by row values [OK]

Common Mistakes:

• Thinking CASE creates new columns
• Confusing CASE with ORDER BY
• Assuming CASE controls JOIN logic

2. Which of the following is the correct syntax to use CASE inside a WHERE clause?

easy

A. WHERE CASE age WHEN > 18 THEN 'adult' ELSE 'minor' END = 'adult'

B. WHERE CASE WHEN age > 18 THEN 'adult' ELSE 'minor' END = 'adult'

C. WHERE CASE WHEN age > 18 THEN adult ELSE minor END

D. WHERE CASE age > 18 THEN 'adult' ELSE 'minor' END = 'adult'

Solution

1. Step 1: Review correct CASE syntax

   The correct CASE syntax uses WHEN condition THEN result ELSE result END. WHERE CASE WHEN age > 18 THEN 'adult' ELSE 'minor' END = 'adult' follows this exactly.

2. Step 2: Identify syntax errors in other options

   Options B, C, and D misuse CASE syntax by missing WHEN or ELSE keywords or incorrect placement.

3. Final Answer:

   WHERE CASE WHEN age > 18 THEN 'adult' ELSE 'minor' END = 'adult' -> Option B

4. Quick Check:

   CASE WHEN ... THEN ... ELSE ... END [OK]

Hint: CASE needs WHEN and THEN keywords in WHERE [OK]

Common Mistakes:

• Omitting WHEN keyword
• Using CASE without END
• Placing conditions incorrectly

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

SELECT * FROM Employees WHERE CASE WHEN salary >= 50000 THEN 1 ELSE 0 END = 1;

medium

A. Rows where salary is less than 50000

B. Rows where salary is exactly 50000

C. All rows regardless of salary

D. Rows where salary is 50000 or more

Solution

1. Step 1: Understand the CASE condition

   The CASE returns 1 if salary is 50000 or more, else 0.

2. Step 2: Apply the WHERE filter

   The WHERE clause keeps rows where CASE result equals 1, so salary must be at least 50000.

3. Final Answer:

   Rows where salary is 50000 or more -> Option D

4. Quick Check:

   salary >= 50000 = filtered rows [OK]

Hint: CASE returns 1 for salary ≥ 50000, filter keeps those rows [OK]

Common Mistakes:

• Thinking it returns rows below 50000
• Confusing equality with greater than
• Ignoring ELSE result

4. Identify the error in this query using CASE in WHERE clause:

SELECT * FROM Orders WHERE CASE order_status WHEN 'shipped' THEN TRUE ELSE FALSE;

medium

A. Missing END keyword for CASE expression

B. Incorrect use of TRUE/FALSE in CASE

C. CASE cannot be used in WHERE clause

D. Missing THEN keyword in CASE

Solution

1. Step 1: Check CASE syntax completeness

   The CASE expression must end with END keyword, which is missing here.

2. Step 2: Verify other parts

   TRUE/FALSE usage is valid, CASE can be used in WHERE, and THEN keyword is present.

3. Final Answer:

   Missing END keyword for CASE expression -> Option A

4. Quick Check:

   CASE ... END required [OK]

Hint: Always close CASE with END in WHERE [OK]

Common Mistakes:

• Forgetting END keyword
• Thinking CASE can't be in WHERE
• Misplacing THEN keyword

5. You want to filter Products so that if category is 'Electronics', only products with stock > 0 are shown; otherwise, show products with price < 100. Which WHERE clause using CASE is correct?

hard

A. WHERE CASE WHEN category = 'Electronics' THEN stock > 0 ELSE price < 100 END = TRUE

B. WHERE CASE category WHEN 'Electronics' THEN stock > 0 ELSE price < 100 = TRUE

C. WHERE CASE WHEN category = 'Electronics' THEN stock > 0 ELSE price < 100

D. WHERE CASE category WHEN 'Electronics' THEN stock > 0 ELSE price < 100

Solution

1. Step 1: Understand CASE returns boolean condition

   The CASE must return TRUE or FALSE to filter rows properly.

2. Step 2: Check syntax and comparison

   WHERE CASE WHEN category = 'Electronics' THEN stock > 0 ELSE price < 100 END = TRUE correctly uses WHEN condition THEN boolean expression ELSE boolean expression END = TRUE to filter rows.

3. Step 3: Identify why others are wrong

   Options A, B, and D miss the END keyword for the CASE expression, causing syntax errors.

4. Final Answer:

   WHERE CASE WHEN category = 'Electronics' THEN stock > 0 ELSE price < 100 END = TRUE -> Option A

5. Quick Check:

   CASE returns boolean, compare with TRUE in WHERE [OK]

Hint: Compare CASE result to TRUE in WHERE for boolean filters [OK]

Common Mistakes:

• Missing END keyword
• Using CASE with simple WHEN without condition
• Returning non-boolean values in CASE