Error handling in procedures in MySQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of a procedure with DECLARE CONTINUE HANDLER

Consider the following MySQL procedure. What will be the output when calling CALL test_error_handling();?

MySQL

DELIMITER $$
CREATE PROCEDURE test_error_handling()
BEGIN
  DECLARE CONTINUE HANDLER FOR SQLEXCEPTION
  BEGIN
    SELECT 'Error caught';
  END;
  -- This will cause a division by zero error
  SELECT 10 / 0;
  SELECT 'No error';
END$$
DELIMITER ;

A Error caught

B Procedure completes without output

C No error

D Syntax error in procedure

💡 Hint

Think about what DECLARE CONTINUE HANDLER does when an error occurs inside the procedure.

The DECLARE CONTINUE HANDLER catches the division by zero error and runs the handler block, which outputs 'Error caught'.

🧠 Conceptual

intermediate

Purpose of DECLARE EXIT HANDLER in MySQL procedures

What is the main difference between a DECLARE CONTINUE HANDLER and a DECLARE EXIT HANDLER in MySQL stored procedures?

A Both handlers behave the same and continue execution after handling the error.

B CONTINUE HANDLER stops the procedure after handling the error; EXIT HANDLER allows it to continue.

C EXIT HANDLER is used only for warnings, CONTINUE HANDLER only for errors.

D CONTINUE HANDLER allows the procedure to continue after handling the error; EXIT HANDLER stops the procedure after handling the error.

💡 Hint

Think about what happens to the procedure flow after the handler runs.

A CONTINUE HANDLER lets the procedure continue after the handler code runs, while an EXIT HANDLER causes the procedure to exit immediately after the handler code executes.

📝 Syntax

advanced

Identify the syntax error in this error handler declaration

Which option correctly fixes the syntax error in this MySQL procedure snippet?

MySQL

CREATE PROCEDURE sample_proc()
BEGIN
  DECLARE HANDLER FOR SQLEXCEPTION
  BEGIN
    SELECT 'Error';
  END;
END;

A Remove BEGIN and END inside handler block

B Add EXIT keyword: DECLARE EXIT HANDLER FOR SQLEXCEPTION

C Change to DECLARE CONTINUE HANDLER FOR SQLEXCEPTION

D No change needed, syntax is correct

💡 Hint

MySQL requires a handler type keyword before HANDLER.

MySQL requires the handler type (CONTINUE or EXIT) before HANDLER. Omitting it causes a syntax error.

🔧 Debug

advanced

Why does this procedure not catch the error as expected?

Given this procedure, why does the error not get caught by the handler?

MySQL

DELIMITER $$
CREATE PROCEDURE test_proc()
BEGIN
  DECLARE EXIT HANDLER FOR SQLEXCEPTION
  BEGIN
    SELECT 'Error caught';
  END;
  SET @x = 1/0;
  SELECT 'No error';
END$$
DELIMITER ;

A The handler is EXIT type, so it exits before printing 'Error caught'

B Division by zero in SET statement does not raise SQLEXCEPTION

C SET @x = 1/0 is evaluated outside the procedure context, so handler does not catch it

D The error occurs before the handler is declared

💡 Hint

Consider where the error happens and how MySQL evaluates expressions.

Division by zero in a SET statement assigning to a user variable raises only a warning (SQLWARNING), not SQLEXCEPTION, so the handler does not trigger.

❓ optimization

expert

Optimizing error handling for multiple error codes

You want to catch multiple specific SQL error codes in a MySQL procedure and handle them differently. Which approach is the most efficient and correct?

A Declare multiple handlers, one for each error code, each with its own handler block

B Use a single EXIT handler for all errors and re-raise errors for unhandled codes

C Declare one handler for SQLWARNING and ignore error codes

D Declare one handler for SQLEXCEPTION and use IF statements inside to check error codes

💡 Hint

MySQL allows multiple handlers for different conditions.

Declaring multiple handlers for specific error codes lets you handle each error precisely and efficiently without extra checks inside the handler.

Practice

1. What is the main purpose of using DECLARE HANDLER in a MySQL stored procedure?

easy

A. To declare variables inside the procedure

B. To catch and handle errors during procedure execution

C. To create temporary tables for data processing

D. To optimize query performance automatically

Solution

1. Step 1: Understand the role of DECLARE HANDLER

   DECLARE HANDLER is used to define actions when certain conditions or errors occur during procedure execution.

2. Step 2: Identify the purpose in error handling

   It helps catch errors and decide whether to continue or exit, preventing crashes and allowing custom responses.

3. Final Answer:

   To catch and handle errors during procedure execution -> Option B

4. Quick Check:

   Error handling = DECLARE HANDLER usage [OK]

Hint: DECLARE HANDLER is for managing errors inside procedures [OK]

Common Mistakes:

• Confusing DECLARE HANDLER with variable declaration
• Thinking it creates tables or indexes
• Assuming it optimizes queries automatically

2. Which of the following is the correct syntax to declare a handler for SQL exceptions that continues execution in MySQL?

easy

A. DECLARE CONTINUE HANDLER FOR SQLEXCEPTION BEGIN ... END;

B. DECLARE HANDLER CONTINUE FOR SQLEXCEPTION;

C. DECLARE HANDLER FOR CONTINUE SQLEXCEPTION;

D. DECLARE HANDLER FOR SQLEXCEPTION CONTINUE;

Solution

1. Step 1: Recall correct DECLARE HANDLER syntax

   The syntax is: DECLARE [CONTINUE|EXIT] HANDLER FOR condition statement;

2. Step 2: Match syntax with options

   DECLARE CONTINUE HANDLER FOR SQLEXCEPTION BEGIN ... END; correctly uses DECLARE CONTINUE HANDLER FOR SQLEXCEPTION followed by a statement block.

3. Final Answer:

   DECLARE CONTINUE HANDLER FOR SQLEXCEPTION BEGIN ... END; -> Option A

4. Quick Check:

   Correct syntax = DECLARE CONTINUE HANDLER FOR SQLEXCEPTION BEGIN ... END; [OK]

Hint: Syntax: DECLARE CONTINUE HANDLER FOR error BEGIN...END [OK]

Common Mistakes:

• Placing CONTINUE after HANDLER incorrectly
• Omitting BEGIN...END block for multiple statements
• Wrong order of keywords

3. Consider this procedure snippet:

DECLARE EXIT HANDLER FOR SQLEXCEPTION SET @error_flag = 1;
SET @error_flag = 0;
INSERT INTO users(id, name) VALUES(1, 'Alice');
INSERT INTO users(id, name) VALUES(1, 'Bob');

Assuming id is a primary key, what will be the value of @error_flag after execution?

medium

A. 1

B. 0

C. NULL

D. Procedure will crash without setting @error_flag

Solution

1. Step 1: Analyze the inserts and handler

   The first insert succeeds. The second insert violates primary key (duplicate id), causing an SQLEXCEPTION.

2. Step 2: Understand EXIT HANDLER effect

   The EXIT HANDLER sets @error_flag = 1 and stops further execution.

3. Final Answer:

   1 -> Option A

4. Quick Check:

   Duplicate key error triggers EXIT HANDLER = 1 [OK]

Hint: EXIT HANDLER sets flag on error and stops execution [OK]

Common Mistakes:

• Assuming @error_flag stays 0 despite error
• Thinking procedure continues after EXIT HANDLER
• Confusing CONTINUE and EXIT handlers

4. Identify the error in this MySQL procedure snippet:

DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;
DECLARE done BOOLEAN DEFAULT FALSE;
WHILE done = FALSE DO
  -- some code
END WHILE;

medium

A. NOT FOUND is not a valid condition for handler

B. Missing semicolon after DECLARE statements

C. WHILE loop condition syntax is incorrect

D. Handler declared before variable 'done' is declared

Solution

1. Step 1: Check declaration order rules

   In MySQL procedures, variables must be declared before handlers.

2. Step 2: Identify declaration order in snippet

   The handler is declared before the variable 'done', which is invalid.

3. Final Answer:

   Handler declared before variable 'done' is declared -> Option D

4. Quick Check:

   Variables before handlers = correct order [OK]

Hint: Declare variables before handlers in procedures [OK]

Common Mistakes:

• Declaring handlers before variables
• Confusing NOT FOUND condition usage
• Ignoring declaration order rules

5. You want to write a procedure that tries to insert a new user but if a duplicate key error occurs, it should update the user's name instead. Which error handling approach is best?

hard

A. Use DECLARE CONTINUE HANDLER for NOT FOUND to insert new user

B. Use DECLARE EXIT HANDLER for any error to rollback transaction

C. Use DECLARE CONTINUE HANDLER for duplicate key error to run UPDATE statement

D. Use no handler and rely on client application to handle errors

Solution

1. Step 1: Understand the goal

   Insert user; if duplicate key error occurs, update existing user's name instead.

2. Step 2: Choose handler type

   CONTINUE HANDLER allows procedure to continue after catching duplicate key error and run UPDATE.

3. Step 3: Evaluate other options

   EXIT HANDLER stops execution, NOT FOUND is unrelated, and no handler defers error handling outside procedure.

4. Final Answer:

   Use DECLARE CONTINUE HANDLER for duplicate key error to run UPDATE statement -> Option C

5. Quick Check:

   CONTINUE HANDLER for duplicate key = update fallback [OK]

Hint: Use CONTINUE HANDLER to catch duplicate key and update [OK]

Common Mistakes:

• Using EXIT HANDLER which stops procedure
• Using NOT FOUND for duplicate key errors
• Ignoring error handling inside procedure