Simple CASE syntax in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to return 'High' when score is 90, else 'Other'.

SQL

SELECT CASE score WHEN [1] THEN 'High' ELSE 'Other' END AS grade FROM exams;

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

A90

B100

C70

D80

Attempts:

3 left

💡 Hint

Common Mistakes

Using a value other than 90 in WHEN clause.

Confusing CASE syntax with searched CASE.

The CASE expression compares score to 90 and returns 'High' if it matches.

2fill in blank

medium

Complete the code to return 'Pass' when status is 'P', else 'Fail'.

SQL

SELECT CASE status WHEN [1] THEN 'Pass' ELSE 'Fail' END AS result FROM tests;

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

A'P'

B'F'

C'X'

D'A'

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'F' which means fail instead of 'P'.

Omitting quotes around the character.

The CASE checks if status equals 'P' to return 'Pass'.

3fill in blank

hard

Fix the error in the CASE expression to return 'Adult' when age is 18.

SQL

SELECT CASE age WHEN [1] THEN 'Adult' ELSE 'Minor' END AS category FROM people;

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

A'18'

B'eighteen'

C18

Dage

Attempts:

3 left

💡 Hint

Common Mistakes

Putting 18 in quotes making it a string.

Using a variable name instead of a value.

Since age is a number, WHEN should compare to 18 without quotes.

4fill in blank

hard

Fill both blanks to return 'Low', 'Medium', or 'High' based on score.

SQL

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

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

A50

B75

C100

D25

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping low and high values.

Using values not in options.

Score 25 returns 'Low', 100 returns 'High', others 'Medium'.

5fill in blank

hard

Fill all three blanks to return 'Red', 'Green', or 'Blue' based on color_code.

SQL

SELECT CASE color_code WHEN [1] THEN 'Red' WHEN [2] THEN 'Green' WHEN [3] THEN 'Blue' ELSE 'Unknown' END AS color_name FROM palette;

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

A1

B2

C3

D4

Attempts:

3 left

💡 Hint

Common Mistakes

Using 4 which is not mapped.

Mixing up the color codes.

Color codes 1, 2, 3 correspond to Red, Green, Blue respectively.

Practice

1. What does the Simple CASE expression do in SQL?

easy

A. It loops through rows and updates values conditionally.

B. It compares one expression to multiple values and returns a result for the first match.

C. It creates a new table based on conditions.

D. It deletes rows that match a condition.

Solution

1. Step 1: Understand the purpose of Simple CASE

   Simple CASE compares one expression against multiple possible values.

2. Step 2: Identify the result behavior

   It returns the result for the first matching value it finds.

3. Final Answer:

   It compares one expression to multiple values and returns a result for the first match. -> Option B

4. Quick Check:

   Simple CASE = compare expression to values [OK]

Hint: Simple CASE matches one value to many options [OK]

Common Mistakes:

• Thinking it loops through rows
• Confusing with IF statements
• Assuming it creates or deletes tables

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

easy

A. CASE expression WHEN value THEN result ELSE default END

B. IF expression THEN result ELSE default END

C. CASE WHEN expression THEN result END

D. SWITCH expression WHEN value THEN result END

Solution

1. Step 1: Recall Simple CASE syntax structure

   The correct syntax is: CASE expression WHEN value THEN result ELSE default END.

2. Step 2: Compare options

   CASE expression WHEN value THEN result ELSE default END matches this syntax exactly, others use incorrect keywords or structure.

3. Final Answer:

   CASE expression WHEN value THEN result ELSE default END -> Option A

4. Quick Check:

   Simple CASE syntax = CASE expr WHEN val THEN res ELSE def END [OK]

Hint: Simple CASE uses 'CASE expr WHEN val THEN res' format [OK]

Common Mistakes:

• Using WHEN without expression after CASE
• Confusing with IF or SWITCH syntax
• Omitting END keyword

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

SELECT EmployeeName,
 CASE DepartmentID
   WHEN 1 THEN 'HR'
   WHEN 2 THEN 'Sales'
   ELSE 'Other'
 END AS DeptName
FROM Employees;

medium

A. Employee names with department names 'HR', 'Sales', or 'Other' based on DepartmentID.

B. Only employees from HR and Sales departments.

C. An error because ELSE is missing.

D. Employee names with numeric DepartmentID values.

Solution

1. Step 1: Understand the CASE expression in the query

   The CASE checks DepartmentID: 1 returns 'HR', 2 returns 'Sales', others return 'Other'.

2. Step 2: Analyze the SELECT output

   The query returns EmployeeName and DeptName with the mapped department names or 'Other'.

3. Final Answer:

   Employee names with department names 'HR', 'Sales', or 'Other' based on DepartmentID. -> Option A

4. Quick Check:

   CASE maps DepartmentID to names [OK]

Hint: CASE returns first matching value or ELSE default [OK]

Common Mistakes:

• Thinking ELSE is required (it's optional)
• Assuming only HR and Sales appear
• Expecting numeric DepartmentID instead of names

4. Identify the error in this Simple CASE query:

SELECT ProductName,
 CASE ProductCategory
   WHEN 'A' THEN 'Category A'
   WHEN 'B' 'Category B'
   ELSE 'Other'
 END AS CategoryName
FROM Products;

medium

A. CASE expression should be inside parentheses.

B. Missing END keyword at the end.

C. Incorrect use of ELSE keyword.

D. Missing THEN keyword after WHEN 'B'.

Solution

1. Step 1: Review each WHEN clause syntax

   WHEN clauses must have THEN keyword before the result. The second WHEN is missing THEN.

2. Step 2: Check other parts of the query

   END keyword is present, ELSE is correctly used, parentheses are not required around CASE expression.

3. Final Answer:

   Missing THEN keyword after WHEN 'B'. -> Option D

4. Quick Check:

   WHEN must be followed by THEN [OK]

Hint: Each WHEN must be followed by THEN [OK]

Common Mistakes:

• Forgetting THEN after WHEN
• Thinking END is optional
• Adding unnecessary parentheses

5. You want to categorize sales amounts into 'Low', 'Medium', and 'High' using Simple CASE. Which query correctly assigns:
- 'Low' for 0
- 'Medium' for 1
- 'High' for 2 or more (default)?

SELECT SaleID,
 CASE SaleAmount
   WHEN 0 THEN 'Low'
   WHEN 1 THEN 'Medium'
   ELSE 'High'
 END AS SaleCategory
FROM Sales;

hard

A. Fails because ELSE cannot be used for multiple values.

B. Incorrect because Simple CASE cannot handle ranges like 2 or more.

C. Correctly categorizes sales as described.

D. Syntax error due to missing WHEN for 2.

Solution

1. Step 1: Understand Simple CASE limitations

   Simple CASE matches exact values; ELSE handles all other cases.

2. Step 2: Analyze the query logic

   WHEN 0 returns 'Low', WHEN 1 returns 'Medium', ELSE covers 2 or more as 'High'.

3. Final Answer:

   Correctly categorizes sales as described. -> Option C

4. Quick Check:

   ELSE covers all unmatched values [OK]

Hint: Use ELSE for all other values in Simple CASE [OK]

Common Mistakes:

• Thinking ELSE can't cover multiple values
• Trying to use ranges in Simple CASE
• Expecting WHEN for every possible value