Raising Error Conditions with MySQL SIGNAL / RESIGNAL Statements

 Raising Error Conditions with MySQL SIGNAL / RESIGNAL Statements

Summary: in this tutorial, you will learn how to use SIGNAL  and RESIGNAL statements to raise error conditions inside stored procedures.

MySQL SIGNAL statement

You use the SIGNAL statement to return an error or warning condition to the caller from a stored program e.g., stored procedure, stored function, trigger, or event. The SIGNAL  statement provides you with control over which information for returning such as value and messageSQLSTATE.

The following illustrates the syntax of the SIGNAL statement:

SIGNAL SQLSTATE | condition_name;
SET condition_information_item_name_1 = value_1,
    condition_information_item_name_1 = value_2, etc;

Following the SIGNAL keyword is a SQLSTATE value or a condition name declared by the  DECLARE CONDITION statement. Notice that the SIGNAL the statement must always specify a SQLSTATE value or a named condition that defined with an  SQLSTATE value.

To provide the caller with information, you use the SET clause. If you want to return multiple condition information item names with values, you need to separate each name/value pair by a comma.

The  condition_information_item_name can be MESSAGE_TEXT, MYSQL_ERRORNO, CURSOR_NAME , etc.

The following stored procedure adds an order line item into an existing sales order. It issues an error message if the order number does not exist.

DELIMITER $$

CREATE PROCEDURE AddOrderItem(
		         in orderNo int,
			 in productCode varchar(45),
			 in qty int, 
                         in price double, 
                         in lineNo int )
BEGIN
	DECLARE C INT;

	SELECT COUNT(orderNumber) INTO C
	FROM orders 
	WHERE orderNumber = orderNo;

	-- check if orderNumber exists
	IF(C != 1) THEN 
		SIGNAL SQLSTATE '45000'
			SET MESSAGE_TEXT = 'Order No not found in orders table';
	END IF;
	-- more code below
	-- ...
END

First, it counts the orders with the input order number that we pass to the stored procedure.

Second, if the number of orders is not 1, it raises an error   SQLSTATE 45000 along with an error message saying that the order number does not exist in the orders table.

Notice that 45000 is a generic SQLSTATE value that illustrates an unhandled user-defined exception.

If we call the stored procedure  AddOrderItem() and pass a nonexistent order number, we will get an error message.

CALL AddOrderItem(10,'S10_1678',1,95.7,1);

MySQL RESIGNAL statement

Besides the SIGNAL  statement, MySQL also provides the RESIGNAL  statement used to raise a warning or error condition.

The RESIGNAL  the statement is similar to SIGNAL  the statement in terms of functionality and syntax, except that:

• You must use the RESIGNAL  a statement within an error or warning handler, otherwise, you will get an error message saying that “RESIGNAL when the handler is not active”. Notice that you can use SIGNAL  statement anywhere inside a stored procedure.
• You can omit all attributes of the RESIGNAL the statement, even the SQLSTATE value.

If you use the RESIGNAL the statement alone, all attributes are the same as the ones passed to the condition handler.

The following stored procedure changes the error message before issuing it to the caller.

DELIMITER $$

CREATE PROCEDURE Divide(IN numerator INT, IN denominator INT, OUT result double)
BEGIN
	DECLARE division_by_zero CONDITION FOR SQLSTATE '22012';

	DECLARE CONTINUE HANDLER FOR division_by_zero 
	RESIGNAL SET MESSAGE_TEXT = 'Division by zero / Denominator cannot be zero';
	-- 
	IF denominator = 0 THEN
		SIGNAL division_by_zero;
	ELSE
		SET result := numerator / denominator;
	END IF;
END

Let’s call the  Divide() stored procedure.

CALL Divide(10,0,@result);

In this tutorial, we have shown you how to raise error conditions inside stored programs using SIGNAL    RESIGNAL statements.