Why CASE expressions are needed in SQL - See It in Action

Why CASE expressions are needed

📖 Scenario: You work in a retail store database. You have a table of sales with product names and quantities sold. You want to label each sale as 'Low', 'Medium', or 'High' quantity based on the number sold.

🎯 Goal: Build a SQL query that uses a CASE expression to categorize sales quantities into 'Low', 'Medium', or 'High'.

📋 What You'll Learn

Create a table called sales with columns product (text) and quantity (integer).

Insert these exact rows into sales: ('Apples', 5), ('Bananas', 15), ('Cherries', 25).

Write a SQL query selecting product, quantity, and a new column quantity_label using a CASE expression.

The CASE expression should label quantity as 'Low' if less than 10, 'Medium' if between 10 and 20 inclusive, and 'High' if greater than 20.

💡 Why This Matters

🌍 Real World

Retail and sales databases often need to classify data dynamically, like labeling sales volumes or customer ratings.

💼 Career

Understanding CASE expressions is essential for data analysts and database developers to write flexible and readable SQL queries.

1

Create the sales table and insert data

Create a table called sales with columns product (text) and quantity (integer). Then insert these rows exactly: ('Apples', 5), ('Bananas', 15), ('Cherries', 25).

SQL

-- Create table sales and insert rows
-- Your code here

Hint

Use CREATE TABLE to define the table and INSERT INTO to add rows.

2

Write the SELECT query skeleton

Write a SQL SELECT query that selects product and quantity from the sales table.

SQL

SELECT product, quantity
FROM sales;
-- Your code here

Hint

Use SELECT product, quantity FROM sales to get the basic data.

3

Add the CASE expression to label quantities

Modify the SELECT query to add a new column called quantity_label using a CASE expression. Label quantity as 'Low' if less than 10, 'Medium' if between 10 and 20 inclusive, and 'High' if greater than 20.

SQL

SELECT product, quantity,
  CASE
    WHEN quantity < 10 THEN 'Low'
    WHEN quantity BETWEEN 10 AND 20 THEN 'Medium'
    ELSE 'High'
  END AS quantity_label
FROM sales;
-- Your code here

Hint

Use CASE WHEN ... THEN ... ELSE ... END AS quantity_label to create the labels.

4

Complete the query with proper formatting

Ensure the full SQL query selects product, quantity, and the quantity_label column with the CASE expression exactly as specified.

SQL

-- Verify the query is complete and formatted correctly
-- Your code here

Hint

Check that the query includes all parts and is syntactically correct.

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