CASE expression in PostgreSQL

Introduction

The CASE expression helps you choose different results based on conditions, like making decisions in your data.

You want to show different labels for numbers, like 'Low', 'Medium', or 'High' based on scores.

You need to replace NULL values with a default text in your query results.

You want to group data into categories directly in your query output.

You want to perform simple if-then-else logic inside a SELECT statement.

You want to create custom sorting or filtering based on conditions.

Syntax

PostgreSQL

CASE
  WHEN condition1 THEN result1
  WHEN condition2 THEN result2
  ...
  ELSE result_default
END

The CASE expression checks each WHEN condition in order and returns the result for the first true condition.

The ELSE part is optional; if no conditions match and ELSE is missing, the result is NULL.

Examples

This example assigns a performance label based on the score value.

PostgreSQL

SELECT
  CASE
    WHEN score >= 90 THEN 'Excellent'
    WHEN score >= 75 THEN 'Good'
    ELSE 'Needs Improvement'
  END AS performance
FROM students;

This example groups people into 'Minor' or 'Adult' based on their age.

PostgreSQL

SELECT
  name,
  CASE
    WHEN age < 18 THEN 'Minor'
    ELSE 'Adult'
  END AS age_group
FROM people;

This example checks if the price is missing and labels it accordingly.

PostgreSQL

SELECT
  product,
  price,
  CASE
    WHEN price IS NULL THEN 'Price not available'
    ELSE 'Price available'
  END AS price_status
FROM products;

Sample Program

This query creates a sales table, inserts some amounts, and uses CASE to categorize each sale by amount.

PostgreSQL

CREATE TABLE sales (
  id SERIAL PRIMARY KEY,
  amount INTEGER
);

INSERT INTO sales (amount) VALUES (50), (150), (250), (NULL);

SELECT
  id,
  amount,
  CASE
    WHEN amount IS NULL THEN 'No Sale'
    WHEN amount < 100 THEN 'Small Sale'
    WHEN amount BETWEEN 100 AND 200 THEN 'Medium Sale'
    ELSE 'Large Sale'
  END AS sale_category
FROM sales
ORDER BY id;

Important Notes

CASE expressions can be used in SELECT, WHERE, ORDER BY, and other clauses.

You can nest CASE expressions inside each other for more complex logic.

Always test your CASE logic with sample data to ensure it behaves as expected.

Summary

CASE lets you return different results based on conditions in your query.

It works like simple if-then-else logic inside SQL.

Use WHEN for conditions and ELSE for a default result if no conditions match.

Practice

1. What does the CASE expression do in PostgreSQL?

easy

A. Returns different results based on conditions

B. Creates a new table

C. Deletes rows from a table

D. Updates data in a table

Solution

1. Step 1: Understand the purpose of CASE

   The CASE expression evaluates conditions and returns a result based on which condition is true.

2. Step 2: Compare with other SQL commands

   Unlike CREATE, DELETE, or UPDATE, CASE does not change data but returns values conditionally.

3. Final Answer:

   Returns different results based on conditions -> Option A

4. Quick Check:

   CASE = conditional results [OK]

Hint: CASE returns values based on conditions, like if-then-else [OK]

Common Mistakes:

• Thinking CASE modifies data
• Confusing CASE with table operations
• Ignoring ELSE for default results

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

easy

A. CASE IF condition THEN result ELSE default END

B. CASE WHEN condition THEN result ELSE default END

C. CASE condition THEN result ELSE default

D. CASE WHEN condition THEN result ELSE default

Solution

1. Step 1: Recall correct CASE syntax

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

2. Step 2: Identify syntax errors in options

   The syntax with 'CASE IF ... END' uses 'IF' instead of 'WHEN'. Syntaxes omitting 'END' (such as 'CASE WHEN ... default' or 'CASE condition ...') are invalid. Only the full syntax is correct.

3. Final Answer:

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

4. Quick Check:

   CASE syntax requires WHEN and END [OK]

Hint: CASE always ends with END and uses WHEN for conditions [OK]

Common Mistakes:

• Using IF instead of WHEN
• Forgetting END at the end
• Omitting ELSE clause

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 salary_level FROM employees;

medium

A. A list of salaries with labels 'High' or 'Low' based on salary value

B. Only salaries greater than 5000

C. Only salaries less than or equal to 5000

D. An error because ELSE is missing

Solution

1. Step 1: Understand the CASE condition

   The CASE checks if salary is greater than 5000; if true, returns 'High', else 'Low'.

2. Step 2: Analyze the SELECT output

   The query returns all salaries with a new column showing 'High' or 'Low' labels accordingly.

3. Final Answer:

   A list of salaries with labels 'High' or 'Low' based on salary value -> Option A

4. Quick Check:

   CASE labels salaries as High or Low [OK]

Hint: CASE adds labels based on conditions without filtering rows [OK]

Common Mistakes:

• Thinking CASE filters rows
• Assuming ELSE is mandatory (it is here but present)
• Confusing output with WHERE clause filtering

4. Identify the error in this CASE expression:

SELECT name, CASE WHEN age > 18 THEN 'Adult' WHEN age < 18 THEN 'Minor' END AS age_group FROM users;

medium

A. Syntax error in SELECT statement

B. Incorrect use of THEN keyword

C. Missing END keyword to close CASE

D. Missing ELSE clause for age = 18

Solution

1. Step 1: Check CASE conditions coverage

   The CASE covers age > 18 and age < 18 but misses age = 18 case.

2. Step 2: Understand ELSE role

   Without ELSE, age = 18 returns NULL. Adding ELSE handles this missing case.

3. Final Answer:

   Missing ELSE clause for age = 18 -> Option D

4. Quick Check:

   CASE needs ELSE for unmatched conditions [OK]

Hint: Add ELSE to handle all unmatched cases in CASE [OK]

Common Mistakes:

• Assuming CASE covers all values without ELSE
• Confusing END keyword placement
• Misusing THEN keyword

5. You want to categorize products by price into 'Cheap' (price < 20), 'Moderate' (20 ≤ price < 100), and 'Expensive' (price ≥ 100). Which CASE expression correctly implements this?

hard

A. CASE WHEN price < 20 THEN 'Cheap' WHEN price >= 20 AND price <= 100 THEN 'Moderate' ELSE 'Expensive' END

B. CASE WHEN price < 20 THEN 'Cheap' WHEN price > 20 AND price < 100 THEN 'Moderate' ELSE 'Expensive' END

C. CASE WHEN price < 20 THEN 'Cheap' WHEN price >= 20 AND price < 100 THEN 'Moderate' ELSE 'Expensive' END

D. CASE WHEN price <= 20 THEN 'Cheap' WHEN price >= 20 AND price < 100 THEN 'Moderate' ELSE 'Expensive' END

Solution

1. Step 1: Analyze price ranges for categories

   'Cheap' is price less than 20, 'Moderate' is price from 20 up to but not including 100, 'Expensive' is 100 or more.

2. Step 2: Check each option's conditions

   CASE WHEN price < 20 THEN 'Cheap' WHEN price >= 20 AND price < 100 THEN 'Moderate' ELSE 'Expensive' END correctly uses < 20, then >= 20 and < 100, else 'Expensive'. CASE WHEN price < 20 THEN 'Cheap' WHEN price > 20 AND price < 100 THEN 'Moderate' ELSE 'Expensive' END misses price = 20 in 'Moderate'. CASE WHEN price <= 20 THEN 'Cheap' WHEN price >= 20 AND price < 100 THEN 'Moderate' ELSE 'Expensive' END overlaps at 20 in both 'Cheap' and 'Moderate'. CASE WHEN price < 20 THEN 'Cheap' WHEN price >= 20 AND price <= 100 THEN 'Moderate' ELSE 'Expensive' END includes 100 in 'Moderate' but should be 'Expensive'.

3. Final Answer:

   CASE WHEN price < 20 THEN 'Cheap' WHEN price >= 20 AND price < 100 THEN 'Moderate' ELSE 'Expensive' END -> Option C

4. Quick Check:

   Use inclusive and exclusive ranges carefully in CASE [OK]

Hint: Use >= and < carefully to avoid overlapping ranges [OK]

Common Mistakes:

• Overlapping or missing ranges
• Using > instead of >= causing gaps
• Misplacing ELSE category boundaries