Error handling in procedures in MySQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of error handling in MySQL stored procedures?

Error handling in MySQL stored procedures helps manage and respond to errors during procedure execution, preventing unexpected crashes and allowing graceful recovery or informative messages.

beginner

Which MySQL statement is used to declare an error handler inside a stored procedure?

The DECLARE HANDLER statement is used to declare an error handler in MySQL stored procedures.

intermediate

What are the two types of handlers you can declare in MySQL procedures?

You can declare CONTINUE and EXIT handlers. CONTINUE runs the handler and continues execution, EXIT runs the handler and exits the block.

intermediate

How do you handle a specific SQL error code in a MySQL procedure?

You declare a handler for that specific error code using DECLARE HANDLER FOR SQLSTATE 'error_code' or DECLARE HANDLER FOR error_number.

beginner

What happens if no handler is declared for an error in a MySQL stored procedure?

If no handler is declared, the error causes the procedure to stop and the error is returned to the caller, which may cause the entire transaction to fail.

Which statement declares an error handler in a MySQL stored procedure?

A DECLARE HANDLER FOR

B CREATE HANDLER

C SET ERROR HANDLER

D BEGIN HANDLER

The correct syntax to declare an error handler is using DECLARE HANDLER FOR.

What does a CONTINUE handler do in a MySQL procedure?

A Stops execution after handling the error

B Rolls back the transaction

C Ignores the error silently

D Continues execution after handling the error

A CONTINUE handler runs the handler code and then continues with the next statement.

How do you specify a handler for all SQL exceptions in MySQL?

A DECLARE HANDLER FOR SQLSTATE 'HY000'

B DECLARE HANDLER FOR SQLEXCEPTION

C DECLARE HANDLER FOR SQLWARNING

D DECLARE HANDLER FOR SQLERROR

SQLEXCEPTION covers all errors and exceptions in MySQL error handling.

What happens if an error occurs and no handler is declared?

A The procedure ignores the error

B The procedure continues silently

C The procedure stops and returns the error

D The procedure retries the statement

Without a handler, the procedure stops and the error is returned to the caller.

Which handler type exits the current block after handling the error in MySQL?

A EXIT

B CONTINUE

C RETRY

D UNDO

EXIT handlers run the handler code and then exit the current code block.

Explain how to declare and use an error handler in a MySQL stored procedure.

Think about how you tell the procedure what to do when an error happens.

Describe the difference between CONTINUE and EXIT handlers in MySQL procedures.

Consider what happens after the handler code runs.

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