CASE in PL/pgSQL in PostgreSQL

Introduction

The CASE statement helps you choose different actions based on conditions inside your PL/pgSQL code. It works like a simple decision maker.

When you want to run different code depending on a variable's value.

When you need to return different results based on conditions inside a function.

When you want to replace multiple IF-ELSE statements with cleaner code.

When you want to categorize data inside a stored procedure.

When you want to handle multiple possible cases in your database logic.

Syntax

PostgreSQL

CASE
  WHEN condition1 THEN result1
  WHEN condition2 THEN result2
  ELSE default_result
END CASE;

The CASE statement checks each WHEN condition in order.

If none of the WHEN conditions match, the ELSE part runs (optional).

Examples

This example assigns letter grades based on a numeric score.

PostgreSQL

CASE
  WHEN score >= 90 THEN 'A'
  WHEN score >= 80 THEN 'B'
  ELSE 'F'
END CASE;

This example uses CASE with a variable to return text based on the day.

PostgreSQL

CASE day_of_week
  WHEN 'Monday' THEN 'Start of week'
  WHEN 'Friday' THEN 'End of workweek'
  ELSE 'Midweek'
END CASE;

Sample Program

This code block sets a score, uses CASE to find the grade, and prints the result.

PostgreSQL

DO $$
DECLARE
  score INTEGER := 85;
  grade TEXT;
BEGIN
  grade := CASE
    WHEN score >= 90 THEN 'A'
    WHEN score >= 80 THEN 'B'
    ELSE 'F'
  END CASE;
  RAISE NOTICE 'Score: %, Grade: %', score, grade;
END $$;

Important Notes

CASE in PL/pgSQL must end with END CASE; inside procedural blocks.

You can use CASE to assign values or control flow inside functions and DO blocks.

Summary

CASE helps you choose actions based on conditions.

It is cleaner than many IF-ELSE statements.

Use END CASE; to close the CASE block in PL/pgSQL.

Practice

1. What is the main purpose of using CASE in PL/pgSQL?

easy

A. To choose different actions based on conditions

B. To create loops that repeat actions

C. To define new tables in the database

D. To permanently store data in variables

Solution

1. Step 1: Understand the role of CASE

   CASE is used to select one action from many based on conditions, like a traffic light deciding when to stop or go.

2. Step 2: Compare with other options

   Loops repeat actions, table creation defines structure, and variables store data, none of which is the main role of CASE.

3. Final Answer:

   To choose different actions based on conditions -> Option A

4. Quick Check:

   CASE chooses actions based on conditions [OK]

Hint: CASE picks actions by conditions, not loops or storage [OK]

Common Mistakes:

• Confusing CASE with loops
• Thinking CASE creates tables
• Assuming CASE stores data permanently

2. Which of the following is the correct way to end a CASE block in PL/pgSQL?

easy

A. END;

B. STOP;

C. FINISH CASE;

D. END CASE;

Solution

1. Step 1: Recall PL/pgSQL syntax for CASE

   In PL/pgSQL, a CASE block must be closed with END CASE; to mark its end clearly.

2. Step 2: Check other options

   END; ends blocks like functions, but CASE specifically needs END CASE;. FINISH CASE; and STOP; are invalid keywords.

3. Final Answer:

   END CASE; -> Option D

4. Quick Check:

   CASE ends with END CASE; [OK]

Hint: Always close CASE with END CASE; in PL/pgSQL [OK]

Common Mistakes:

• Using END; alone to close CASE
• Writing FINISH CASE; which is invalid
• Using STOP; which is not a PL/pgSQL keyword

3. Consider this PL/pgSQL snippet:

DECLARE
  grade CHAR := 'B';
  result TEXT;
BEGIN
  CASE grade
    WHEN 'A' THEN result := 'Excellent';
    WHEN 'B' THEN result := 'Good';
    ELSE result := 'Average';
  END CASE;
  RETURN result;
END;

What will be the returned value?

medium

A. 'Good'

B. 'Average'

C. 'Excellent'

D. NULL

Solution

1. Step 1: Identify the value of grade

   The variable grade is set to 'B'.

2. Step 2: Match grade in CASE

   CASE checks 'B', matches the second WHEN clause, so result becomes 'Good'.

3. Final Answer:

   'Good' -> Option A

4. Quick Check:

   grade 'B' returns 'Good' [OK]

Hint: Match CASE value to WHEN clause for output [OK]

Common Mistakes:

• Choosing ELSE when a WHEN matches
• Confusing variable assignment inside CASE
• Assuming NULL if no ELSE present

4. Identify the error in this PL/pgSQL CASE block:

DECLARE
  score INT := 85;
  grade TEXT;
BEGIN
  CASE
    WHEN score >= 90 THEN grade := 'A';
    WHEN score >= 80 THEN grade := 'B';
    ELSE grade := 'C';
  END;
  RETURN grade;
END;

medium

A. Incorrect variable declaration

B. Invalid comparison operators

C. Missing END CASE; to close CASE block

D. No ELSE clause present

Solution

1. Step 1: Check CASE block ending

   The CASE block is closed with END; but PL/pgSQL requires END CASE; to close CASE.

2. Step 2: Verify other parts

   Variable declarations and comparisons are correct, and ELSE clause is present.

3. Final Answer:

   Missing END CASE; to close CASE block -> Option C

4. Quick Check:

   CASE must end with END CASE; [OK]

Hint: Close CASE with END CASE;, not just END; [OK]

Common Mistakes:

• Using END; instead of END CASE;
• Thinking ELSE is optional here
• Misreading comparison operators

5. You want to write a PL/pgSQL function that returns 'Pass' if a student's score is 50 or more, 'Fail' if below 50, and 'Invalid' if the score is NULL. Which CASE structure correctly implements this?

hard

A. CASE score
WHEN NULL THEN RETURN 'Invalid';
WHEN >= 50 THEN RETURN 'Pass';
ELSE RETURN 'Fail';
END CASE;

B. CASE
WHEN score IS NULL THEN RETURN 'Invalid';
WHEN score >= 50 THEN RETURN 'Pass';
ELSE RETURN 'Fail';
END CASE;

C. CASE
WHEN score >= 50 THEN RETURN 'Pass';
WHEN score IS NULL THEN RETURN 'Fail';
ELSE RETURN 'Invalid';
END CASE;

D. CASE score
WHEN score >= 50 THEN RETURN 'Pass';
WHEN score < 50 THEN RETURN 'Fail';
ELSE RETURN 'Invalid';
END CASE;

Solution

1. Step 1: Handle NULL explicitly

   Since NULL cannot be matched by simple WHEN, use WHEN score IS NULL to check NULL values.

2. Step 2: Order conditions correctly

   Check NULL first, then score >= 50 for 'Pass', else 'Fail'. This matches

   CASE
     WHEN score IS NULL THEN RETURN 'Invalid';
     WHEN score >= 50 THEN RETURN 'Pass';
     ELSE RETURN 'Fail';
   END CASE;

   .

3. Final Answer:

   CASE with WHEN score IS NULL, then score >= 50, else Fail -> Option B

4. Quick Check:

   Use IS NULL to check NULL in CASE [OK]

Hint: Use WHEN score IS NULL to test NULL in CASE [OK]

Common Mistakes:

• Trying to match NULL with WHEN NULL
• Using CASE score with conditions inside WHEN
• Not checking NULL before other conditions