Exception handling (BEGIN-EXCEPTION-END) in PostgreSQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to start a block for exception handling in PostgreSQL.

PostgreSQL

BEGIN[1]

Drag options to blanks, or click blank then click option'

ADECLARE

BEND

CBEGIN

DEXCEPTION

Attempts:

3 left

💡 Hint

Common Mistakes

Using EXCEPTION to start the block instead of BEGIN.

Using END to start the block.

Using DECLARE to start the block.

The BEGIN keyword starts a block where you can write SQL statements and handle exceptions.

2fill in blank

medium

Complete the code to catch exceptions in a PostgreSQL block.

PostgreSQL

BEGIN
  -- some code
[1]
  WHEN others THEN
    RAISE NOTICE 'Error occurred';
END;

Drag options to blanks, or click blank then click option'

AEXCEPTION

BIF

CDECLARE

DTRY

Attempts:

3 left

💡 Hint

Common Mistakes

Using TRY instead of EXCEPTION.

Using IF instead of EXCEPTION.

Using DECLARE instead of EXCEPTION.

The EXCEPTION keyword starts the exception handling section inside a block.

3fill in blank

hard

Fix the error in the exception block keyword.

PostgreSQL

BEGIN
  -- some code
[1]
  WHEN division_by_zero THEN
    RAISE NOTICE 'Cannot divide by zero';
END;

Drag options to blanks, or click blank then click option'

AEXCEPTION

BEXCEPT

CCATCH

DHANDLE

Attempts:

3 left

💡 Hint

Common Mistakes

Using EXCEPT instead of EXCEPTION.

Using CATCH or HANDLE which are not valid in PostgreSQL.

The correct keyword to start exception handling in PostgreSQL is EXCEPTION. Other keywords like EXCEPT, CATCH, or HANDLE are not valid here.

4fill in blank

hard

Fill both blanks to complete the exception block that handles a unique violation error.

PostgreSQL

BEGIN
  INSERT INTO users(username) VALUES('admin');
[1]
  WHEN unique_violation THEN
    RAISE NOTICE 'Username already exists';
[2]
END;

Drag options to blanks, or click blank then click option'

AEXCEPTION

BWHEN

CEND

DDECLARE

Attempts:

3 left

💡 Hint

Common Mistakes

Using WHEN to start the exception block instead of EXCEPTION.

Using DECLARE instead of EXCEPTION.

Forgetting to close the block with END.

The EXCEPTION keyword starts the exception handling section, and END closes the block.

5fill in blank

hard

Fill all three blanks to complete the exception block that handles division by zero and others.

PostgreSQL

BEGIN
  PERFORM 10 / [1];
[2]
  WHEN division_by_zero THEN
    RAISE NOTICE 'Division by zero error';
  WHEN others THEN
    RAISE NOTICE 'Some other error';
[3]
END;

Drag options to blanks, or click blank then click option'

A0

BEXCEPTION

CEND

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using 1 instead of 0 as divisor to cause division by zero error.

Using TRY or CATCH instead of EXCEPTION.

Forgetting to close the block with END.

The first blank is the divisor which causes the error (0). The second blank starts the exception block, and the third blank ends the block.

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