CASE in PL/pgSQL in PostgreSQL - Step-by-Step Execution

Concept Flow - CASE in PL/pgSQL

Start

↓

Evaluate CASE expression or conditions

↓

Match WHEN condition?

No→Check next WHEN

Yes↓

Execute THEN result

↓

Exit CASE

↓

If no WHEN matched, execute ELSE (optional)

↓

End CASE

The CASE statement checks conditions one by one and executes the matching THEN part, or ELSE if no match.

Execution Sample

PostgreSQL

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

This code assigns a text result based on the grade value using CASE.

Execution Table

Step	grade	Condition Checked	Condition Result	Action Taken	result
1	'B'	grade = 'A'	False	Check next WHEN	NULL
2	'B'	grade = 'B'	True	result := 'Good'	Good
3	'B'	N/A	N/A	Exit CASE	Good

💡 Condition grade = 'B' matched, so CASE exits after assigning 'Good' to result.

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
grade	'B'	'B'	'B'	'B'
result	NULL	NULL	'Good'	'Good'

Key Moments - 2 Insights

Why does the CASE stop checking conditions after the second WHEN?

Because the condition grade = 'B' is True at step 2 in the execution_table, CASE executes that THEN and exits immediately without checking further.

What happens if none of the WHEN conditions match?

If no WHEN matches, the ELSE part runs if present, as shown in the flow after checking all WHENs with no match.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'result' after step 1?

A 'Excellent'

B 'Good'

C NULL

D 'Needs Improvement'

💡 Hint

Check the 'result' column in execution_table row for step 1.

At which step does the CASE statement exit?

A Step 3

B Step 1

C Step 2

D After ELSE

💡 Hint

Look at the 'Action Taken' column where it says 'Exit CASE'.

If grade was 'C' instead of 'B', what would 'result' be after CASE?

A 'Good'

B 'Needs Improvement'

C 'Excellent'

D NULL

💡 Hint

Refer to the ELSE action in the concept_flow and execution_table logic.

Concept Snapshot

CASE in PL/pgSQL:
- Checks conditions in order
- Executes THEN for first TRUE WHEN
- ELSE runs if no WHEN matches
- Stops checking after first match
- Syntax: CASE expr WHEN val THEN ... ELSE ... END CASE;

Full Transcript

The CASE statement in PL/pgSQL evaluates conditions one by one. It compares the given expression or conditions to each WHEN clause. When it finds a match, it executes the corresponding THEN part and stops checking further. If no WHEN matches, it executes the ELSE part if provided. This helps choose actions based on different values clearly and simply.

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