Error handling in procedures in MySQL - Step-by-Step Execution

Concept Flow - Error handling in procedures

Start Procedure

↓

Execute Statements

↓

Error Occurs?

No→Continue Execution

Yes↓

Handle Error with DECLARE HANDLER

↓

Perform Recovery or Logging

↓

End Procedure

The procedure starts and runs statements. If an error happens, the declared handler catches it and runs recovery code before ending.

Execution Sample

MySQL

CREATE PROCEDURE test_proc()
BEGIN
  DECLARE CONTINUE HANDLER FOR SQLEXCEPTION
  BEGIN
    SET @error_flag = 1;
  END;
  SET @error_flag = 0;
  INSERT INTO test_table VALUES (1);
END;

This procedure tries to insert a row and sets an error flag if an SQL exception occurs.

Execution Table

Step	Action	Evaluation	Result
1	Start procedure	N/A	Procedure begins execution
2	Declare CONTINUE HANDLER for SQLEXCEPTION	N/A	Handler ready to catch SQL errors
3	Set @error_flag = 0	N/A	@error_flag is 0 (no error yet)
4	Execute INSERT INTO test_table VALUES (1)	Success	Row inserted, no error
5	End procedure	N/A	Procedure ends normally

💡 Procedure ends after successful insert without errors

Variable Tracker

Variable	Start	After Step 3	After Step 4	Final
@error_flag	NULL	0	0	0

Key Moments - 2 Insights

Why do we declare a CONTINUE HANDLER before executing statements?

The CONTINUE HANDLER is declared first so it can catch any SQL errors during the statements execution, as shown in step 2 and step 4 of the execution table.

What happens if an error occurs during the INSERT statement?

If an error occurs, the handler runs and sets @error_flag to 1 instead of stopping the procedure, but in this example no error occurs (step 4).

Visual Quiz - 3 Questions

Look at the execution table, what is the value of @error_flag after step 3?

A 1

B 0

C NULL

D Undefined

💡 Hint

Check the variable_tracker table column 'After Step 3' for @error_flag

At which step does the procedure declare the error handler?

A Step 3

B Step 1

C Step 2

D Step 4

💡 Hint

Look at the execution_table 'Action' column for when the handler is declared

If the INSERT caused an error, what would happen to @error_flag?

A It would become 1

B It would become NULL

C It would stay 0

D Procedure would stop immediately

💡 Hint

Refer to the handler code in the execution_sample and key_moments about error handling

Concept Snapshot

Error handling in MySQL procedures:
- Use DECLARE HANDLER to catch errors
- CONTINUE HANDLER lets procedure continue after error
- Handler code runs when specified error occurs
- Use variables to track error state
- Place handler declaration before statements
- Helps avoid procedure stopping unexpectedly

Full Transcript

This visual trace shows how error handling works in MySQL procedures. The procedure starts and declares a CONTINUE HANDLER for SQL exceptions. It sets an error flag variable to zero before running an INSERT statement. If the INSERT succeeds, the flag stays zero and the procedure ends normally. If an error occurs during the INSERT, the handler runs and sets the flag to one, allowing the procedure to continue instead of stopping. This method helps manage errors gracefully inside stored procedures.

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