CASE expression in PostgreSQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the CASE expression in PostgreSQL?

The CASE expression lets you perform conditional logic inside SQL queries, returning different values based on conditions, similar to an IF-THEN-ELSE in programming.

beginner

Write the basic syntax of a simple CASE expression in PostgreSQL.

CASE expression syntax:
CASE input_expression
WHEN value1 THEN result1
WHEN value2 THEN result2
ELSE default_result
END

intermediate

How does a searched CASE expression differ from a simple CASE expression?

A searched CASE tests multiple boolean conditions directly (WHEN condition THEN result), while a simple CASE compares one expression to multiple values.

beginner

What happens if no WHEN condition matches and there is no ELSE clause in a CASE expression?

If no WHEN matches and ELSE is missing, the CASE expression returns NULL.

beginner

Can CASE expressions be used in SELECT, WHERE, and ORDER BY clauses in PostgreSQL?

Yes, CASE expressions can be used anywhere an expression is allowed, including SELECT, WHERE, ORDER BY, and more.

What does the CASE expression return if no WHEN condition matches and ELSE is provided?

A An error

B The first WHEN condition's value

C NULL

D The value in ELSE clause

If no WHEN matches, CASE returns the ELSE value if provided.

Which of the following is a valid use of CASE in PostgreSQL?

A Inside SELECT to change output values

B Only in WHERE clause

C Only in ORDER BY clause

D Only in UPDATE statements

CASE can be used anywhere expressions are allowed, including SELECT.

What keyword starts a searched CASE expression?

A WHEN

B CASE

C IF

D ELSE

All CASE expressions start with the CASE keyword.

In a simple CASE expression, what is compared to the WHEN values?

A The ELSE clause

B Boolean conditions

C An input expression

D The SELECT statement

Simple CASE compares one input expression to each WHEN value.

What is the result of a CASE expression without ELSE and no matching WHEN?

A NULL

B 0

C Error

D First WHEN value

Without ELSE, CASE returns NULL if no WHEN matches.

Explain how to use a searched CASE expression in PostgreSQL with an example.

Think about checking conditions like age > 18 or status = 'active'.

Describe the difference between simple CASE and searched CASE expressions in PostgreSQL.

One compares values, the other checks conditions.

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