Exception handling (BEGIN-EXCEPTION-END) in PostgreSQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the BEGIN-EXCEPTION-END block in PostgreSQL?

It is used to handle errors gracefully by running code that might fail inside BEGIN, catching errors in EXCEPTION, and then continuing or cleaning up in END.

intermediate

How do you catch a specific error in a PostgreSQL exception block?

Use WHEN followed by the error name or SQLSTATE code inside the EXCEPTION block to catch specific errors.

beginner

What happens if an error occurs inside a BEGIN block but there is no EXCEPTION block?

The error will stop the transaction and the error will be returned to the caller without any special handling.

intermediate

Write a simple example of a BEGIN-EXCEPTION-END block that catches division by zero error.

BEGIN PERFORM 1 / 0; EXCEPTION WHEN division_by_zero THEN RAISE NOTICE 'Cannot divide by zero'; END;

intermediate

Can you re-raise an error inside the EXCEPTION block in PostgreSQL?

Yes, you can use RAISE to re-throw the error after handling or logging it.

What keyword starts the block where you handle exceptions in PostgreSQL?

A BEGIN

B EXCEPTION

C TRY

D CATCH

The BEGIN keyword starts the block where you write code that might raise exceptions.

Which keyword is used to catch errors in PostgreSQL exception handling?

A CATCH

B EXCEPTION

C HANDLE

D WHEN

Inside the EXCEPTION block, WHEN is used to specify which errors to catch.

What happens if an error is not caught inside a BEGIN-EXCEPTION-END block?

A The transaction continues normally

B The error is propagated and stops the transaction

C The error is ignored

D The error is automatically fixed

If an error is not caught, it propagates and usually stops the transaction.

How do you handle a division by zero error in PostgreSQL exception block?

A WHEN div_zero THEN

B WHEN zero_division THEN

C WHEN division_by_zero THEN

D WHEN divide_by_zero THEN

The correct error name is division_by_zero.

Can you re-raise an error inside the EXCEPTION block?

A Yes, using RAISE

B Yes, using THROW

C No, errors cannot be re-raised

D Only in PL/pgSQL functions

You can re-raise errors using the RAISE statement.

Explain how the BEGIN-EXCEPTION-END block works in PostgreSQL for error handling.

Think about how you try something, catch problems, and then finish.

Describe how to catch and handle a specific error like division by zero in PostgreSQL.

Focus on the keywords and error name.

Practice

1. What is the main purpose of using a BEGIN-EXCEPTION-END block in PostgreSQL?

easy

A. To catch and handle errors during code execution

B. To speed up query execution

C. To create new tables automatically

D. To backup the database

Solution

1. Step 1: Understand the role of BEGIN-EXCEPTION-END

   This block is designed to catch errors that happen during the execution of SQL or PL/pgSQL code.

2. Step 2: Identify the purpose of error handling

   It allows the code to handle errors gracefully without stopping the entire process.

3. Final Answer:

   To catch and handle errors during code execution -> Option A

4. Quick Check:

   Error handling = A [OK]

Hint: Remember: BEGIN-EXCEPTION-END is for error catching [OK]

Common Mistakes:

• Thinking it speeds up queries
• Confusing it with table creation
• Assuming it backs up data

2. Which of the following is the correct syntax to start an exception block in PostgreSQL?

easy

A. BEGIN TRY ... EXCEPTION ... END

B. START ... EXCEPTION ... FINISH

C. BEGIN ... EXCEPTION ... END

D. TRY ... CATCH ... END

Solution

1. Step 1: Recall PostgreSQL exception block syntax

   PostgreSQL uses BEGIN ... EXCEPTION ... END to define a block with error handling.

2. Step 2: Compare options with correct syntax

   Only BEGIN ... EXCEPTION ... END matches the exact syntax used in PostgreSQL.

3. Final Answer:

   BEGIN ... EXCEPTION ... END -> Option C

4. Quick Check:

   PostgreSQL uses BEGIN-EXCEPTION-END syntax [OK]

Hint: Look for BEGIN and EXCEPTION keywords together [OK]

Common Mistakes:

• Using TRY-CATCH like other languages
• Adding extra keywords like START or FINISH
• Mixing syntax from other SQL dialects

3. What will be the output of the following PostgreSQL block?

DO $$
BEGIN
  RAISE EXCEPTION 'Error happened';
EXCEPTION
  WHEN OTHERS THEN
    RAISE NOTICE 'Caught an error';
END $$;

medium

A. The block raises an error and stops execution

B. It prints 'Caught an error' as a notice and continues

C. It prints 'Error happened' as a notice

D. Syntax error due to missing semicolon

Solution

1. Step 1: Analyze the RAISE EXCEPTION statement

   The block raises an exception with message 'Error happened'.

2. Step 2: Check the EXCEPTION block handling

   The exception handler catches all errors (WHEN OTHERS) and raises a NOTICE with 'Caught an error'.

3. Final Answer:

   It prints 'Caught an error' as a notice and continues -> Option B

4. Quick Check:

   Exception caught triggers NOTICE message [OK]

Hint: WHEN OTHERS catches all errors and can raise NOTICE [OK]

Common Mistakes:

• Assuming the error stops execution
• Confusing EXCEPTION message with NOTICE
• Thinking syntax error occurs

4. Identify the error in this PostgreSQL exception block:

BEGIN
  PERFORM 1/0;
EXCEPTION
  WHEN division_by_zero THEN
    RAISE NOTICE 'Division by zero caught';
END;

medium

A. Missing DO $$ ... $$ wrapper for anonymous block

B. Incorrect exception name; should be zero_division_error

C. RAISE NOTICE cannot be used inside EXCEPTION

D. No error; block is correct

Solution

1. Step 1: Check block context

   In PostgreSQL, standalone BEGIN-EXCEPTION-END blocks must be inside a DO block or function.

2. Step 2: Verify exception name and usage

   Exception name 'division_by_zero' is valid, and RAISE NOTICE is allowed inside EXCEPTION.

3. Step 3: Identify missing DO wrapper

   The code lacks DO $$ ... $$ to run as an anonymous block.

4. Final Answer:

   Missing DO $$ ... $$ wrapper for anonymous block -> Option A

5. Quick Check:

   Anonymous blocks need DO wrapper [OK]

Hint: Use DO $$ ... $$ for anonymous BEGIN blocks [OK]

Common Mistakes:

• Thinking exception name is wrong
• Believing RAISE NOTICE is invalid here
• Ignoring the need for DO wrapper

5. You want to write a PostgreSQL block that tries to insert a row into a table but if a unique constraint violation occurs, it should print 'Duplicate found' and continue without error. Which code snippet correctly implements this?

hard

A. BEGIN
INSERT INTO users(id) VALUES(1);
EXCEPTION
WHEN duplicate_key THEN
RAISE NOTICE 'Duplicate found';
END;

B. BEGIN
INSERT INTO users(id) VALUES(1);
EXCEPTION
WHEN unique_error THEN
RAISE NOTICE 'Duplicate found';
END;

C. BEGIN
INSERT INTO users(id) VALUES(1);
EXCEPTION
WHEN unique_constraint THEN
RAISE NOTICE 'Duplicate found';
END;

D. BEGIN
INSERT INTO users(id) VALUES(1);
EXCEPTION
WHEN unique_violation THEN
RAISE NOTICE 'Duplicate found';
END;

Solution

1. Step 1: Identify correct exception name for unique constraint

   PostgreSQL uses unique_violation to catch unique constraint errors.

2. Step 2: Check the exception block syntax

   The block catches unique_violation and raises a notice 'Duplicate found' correctly.

3. Step 3: Verify other options

   Other options use incorrect exception names that do not exist in PostgreSQL.

4. Final Answer:

   BEGIN INSERT INTO users(id) VALUES(1); EXCEPTION WHEN unique_violation THEN RAISE NOTICE 'Duplicate found'; END; -> Option D

5. Quick Check:

   Use unique_violation for unique constraint errors [OK]

Hint: Use unique_violation to catch duplicate key errors [OK]

Common Mistakes:

• Using wrong exception names
• Forgetting to handle unique_violation
• Not raising a notice or message