CASE in PL/pgSQL in PostgreSQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of CASE expression in PL/pgSQL function

What is the output of the following PL/pgSQL function when called with input value 15?

CREATE OR REPLACE FUNCTION test_case(val INTEGER) RETURNS TEXT AS $$
DECLARE
  result TEXT;
BEGIN
  result := CASE 
    WHEN val < 10 THEN 'Less than 10'
    WHEN val <= 20 THEN 'Between 10 and 20'
    ELSE 'Greater than 20'
  END;
  RETURN result;
END;
$$ LANGUAGE plpgsql;

SELECT test_case(15);

PostgreSQL

CREATE OR REPLACE FUNCTION test_case(val INTEGER) RETURNS TEXT AS $$
DECLARE
  result TEXT;
BEGIN
  result := CASE 
    WHEN val < 10 THEN 'Less than 10'
    WHEN val <= 20 THEN 'Between 10 and 20'
    ELSE 'Greater than 20'
  END;
  RETURN result;
END;
$$ LANGUAGE plpgsql;

SELECT test_case(15);

A 'Greater than 20'

B 'Less than 10'

C 'Between 10 and 20'

D NULL

💡 Hint

Think about which WHEN condition matches the input value 15 first.

The CASE expression checks conditions in order. Since 15 is not less than 10 but is less than or equal to 20, it returns 'Between 10 and 20'.

📝 Syntax

intermediate

Identify the syntax error in CASE statement

Which option contains a syntax error in the CASE statement inside a PL/pgSQL block?

PostgreSQL

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

A Missing semicolon after END of CASE expression

B Missing ELSE clause in CASE expression

C No syntax error; code is correct

D Missing END CASE keyword

💡 Hint

Check if the CASE expression is properly terminated with a semicolon.

In PL/pgSQL, the CASE expression must end with a semicolon after END to complete the assignment statement.

❓ optimization

advanced

Optimizing multiple CASE statements in PL/pgSQL

You have multiple CASE statements checking the same variable in a PL/pgSQL function. Which approach is more efficient?

PostgreSQL

DECLARE
  status INTEGER := 2;
  message TEXT;
BEGIN
  -- Option 1: Multiple CASE statements
  IF status = 1 THEN
    message := 'Start';
  ELSIF status = 2 THEN
    message := 'Processing';
  ELSE
    message := 'Unknown';
  END IF;

  -- Option 2: Single CASE statement
  message := CASE status
    WHEN 1 THEN 'Start'
    WHEN 2 THEN 'Processing'
    ELSE 'Unknown'
  END;
END;

A Option 2 is more efficient because it uses a single CASE expression

B Option 1 is more efficient because IF statements are faster

C Both options have the same efficiency

D Option 1 is more readable and thus preferred

💡 Hint

Consider how many times the variable is evaluated and how concise the code is.

Using a single CASE expression evaluates the variable once and assigns the result directly, which is more efficient and cleaner.

🧠 Conceptual

advanced

Understanding CASE with NULL values in PL/pgSQL

What will be the output of this PL/pgSQL snippet when input_value is NULL?

DECLARE
  input_value INTEGER := NULL;
  output TEXT;
BEGIN
  output := CASE input_value
    WHEN 1 THEN 'One'
    WHEN NULL THEN 'Null'
    ELSE 'Other'
  END;
  RETURN output;
END;

PostgreSQL

DECLARE
  input_value INTEGER := NULL;
  output TEXT;
BEGIN
  output := CASE input_value
    WHEN 1 THEN 'One'
    WHEN NULL THEN 'Null'
    ELSE 'Other'
  END;
  RETURN output;
END;

A Raises an error due to NULL comparison

B 'Null'

C 'One'

D 'Other'

💡 Hint

Remember how NULL comparisons behave in SQL and PL/pgSQL CASE expressions.

In CASE expressions, WHEN NULL does not match NULL because NULL is not equal to anything, even NULL. So ELSE branch is taken.

🔧 Debug

expert

Debugging unexpected CASE behavior in PL/pgSQL

A developer wrote this PL/pgSQL function but it always returns 'Unknown' regardless of input. What is the bug?

CREATE OR REPLACE FUNCTION check_status(code TEXT) RETURNS TEXT AS $$
DECLARE
  result TEXT;
BEGIN
  result := CASE code
    WHEN 'A' THEN 'Active'
    WHEN 'I' THEN 'Inactive'
    ELSE 'Unknown'
  END;
  RETURN result;
END;
$$ LANGUAGE plpgsql;

-- Called as: SELECT check_status('a');

PostgreSQL

CREATE OR REPLACE FUNCTION check_status(code TEXT) RETURNS TEXT AS $$
DECLARE
  result TEXT;
BEGIN
  result := CASE code
    WHEN 'A' THEN 'Active'
    WHEN 'I' THEN 'Inactive'
    ELSE 'Unknown'
  END;
  RETURN result;
END;
$$ LANGUAGE plpgsql;

-- Called as: SELECT check_status('a');

A Variable 'result' is not initialized before assignment

B CASE expression is case-sensitive, so 'a' does not match 'A'

C Function lacks RETURNS clause

D Missing ELSE clause causes default to 'Unknown'

💡 Hint

Check how string comparisons work in CASE expressions.

The CASE expression compares strings exactly, so lowercase 'a' does not match uppercase 'A'.

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