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MySQL Define Conditions And Handlers

In MySQL, a handler is a routine that is invoked when a given condition occurs, such as an error, warning, or note. They are particularly useful within stored procedures and functions, allowing for more sophisticated error handling and control flow.

A condition can be a SQLSTATE, a MySQL error code, or a named condition that you've defined yourself.

Here is a basic example of how to define a condition and a handler in a MySQL stored procedure:

Define the Condition:
A named condition can be defined for a specific MySQL error code or SQLSTATE.
```
DECLARE condition_name CONDITION FOR condition_value;
```
condition_name is the name of the condition that you're defining, and condition_value can be either a MySQL error code or a SQLSTATE.
For example:
```
DECLARE no_data_found CONDITION FOR SQLSTATE '02000';
```
This declares a condition named no_data_found for the SQLSTATE '02000', which corresponds to an error when no data is found.
Define the Handler:
Handlers can be declared to specify what action should be taken when a given condition occurs. The syntax is:
```
DECLARE handler_type HANDLER FOR condition_value statement;
```
handler_type can be CONTINUE, EXIT, or UNDO. condition_value can be a MySQL error code, a SQLSTATE, or a named condition. statement is the SQL statement to execute when the condition occurs.
For example:
```
DECLARE CONTINUE HANDLER FOR no_data_found 
    SET @info = 'No data found';
```
This declares a CONTINUE handler for the no_data_found condition. When no data is found, it sets the @info variable to the string 'No data found'.

Here's a complete example of a stored procedure that uses a condition and a handler:

DELIMITER $$

CREATE PROCEDURE `get_employee`(IN emp_id INT, OUT emp_name VARCHAR(255))
BEGIN
    DECLARE no_data_found CONDITION FOR SQLSTATE '02000';
    DECLARE CONTINUE HANDLER FOR no_data_found 
        SET @info = 'No data found';

    SELECT name INTO emp_name FROM employees WHERE id = emp_id;

    SELECT @info;
END$$

DELIMITER ;

In this stored procedure, if the SELECT statement doesn't find an employee with the given emp_id, it will trigger the no_data_found condition, and the handler will set the @info variable to 'No data found'. The SELECT @info; statement will then display the value of @info, allowing you to see whether the SELECT statement found an employee or not.

Please note that in a real-world application, you might want to use more sophisticated error handling, such as logging the error to a table or raising an error to the caller.

Defining error conditions in MySQL:
- Define error conditions in MySQL using the DECLARE statement. This sets up an error condition that can be checked and handled later.
```
DECLARE custom_error CONDITION FOR SQLSTATE '45000';
```
Handling errors with conditions in MySQL:
- Handle errors with conditions in MySQL using the DECLARE CONTINUE HANDLER statement to specify what action to take when a particular condition is encountered.
```
DECLARE CONTINUE HANDLER FOR SQLSTATE '45000'
    -- Error handling statements here
```
Exception handling in MySQL stored procedures:
- Implement exception handling in MySQL stored procedures using DECLARE CONTINUE HANDLER to catch specific conditions and handle them gracefully.
```
DECLARE CONTINUE HANDLER FOR SQLSTATE '45000'
    -- Exception handling statements here
```
Error codes and handlers in MySQL:
- Use error codes and handlers in MySQL to gracefully handle specific error conditions within stored procedures.
```
DECLARE custom_error CONDITION FOR SQLSTATE '45000';
DECLARE CONTINUE HANDLER FOR custom_error
    -- Error handling statements here
```
Defining custom conditions in MySQL:
- Define custom conditions in MySQL using the DECLARE CONDITION statement. This allows you to create custom error conditions for specific scenarios.
```
DECLARE custom_error CONDITION FOR SQLSTATE '45000';
```
Handling specific errors in MySQL procedures:
- Handle specific errors in MySQL procedures using DECLARE CONTINUE HANDLER for each specific error condition that you want to catch.
```
DECLARE CONTINUE HANDLER FOR SQLSTATE '45000'
    -- Handle custom error
DECLARE CONTINUE HANDLER FOR SQLEXCEPTION
    -- Handle general SQL exception
```
MySQL SIGNAL and RESIGNAL statements:
- Use the SIGNAL and RESIGNAL statements in MySQL to raise a specific error with a custom message and SQLSTATE.
```
SIGNAL SQLSTATE '45000'
    SET MESSAGE_TEXT = 'Custom error message';
```
Using MySQL handlers for graceful error handling:
- Utilize MySQL handlers for graceful error handling by declaring handlers for specific conditions and defining actions to take when those conditions occur.
```
DECLARE CONTINUE HANDLER FOR SQLSTATE '45000'
    -- Handle custom error
DECLARE CONTINUE HANDLER FOR SQLEXCEPTION
    -- Handle general SQL exception
```