Add foreign key constraint if it is not existing - mysql

Question

I want to create a foreign key from 1 table, but only if it does not exist.
Tables are created like that:
CREATE TABLE
IF NOT EXISTS PEs (
id INT(20) AUTO_INCREMENT PRIMARY KEY,
Name varchar(20),
Message varchar(30),
CoordsX double(9,6) SIGNED,
CoordsY double(9,6) SIGNED,
CoordsZ double(9,6) SIGNED,
Status smallint(1) DEFAULT 1,
world varchar(20)
) ENGINE = InnoDB;
CREATE TABLE
IF NOT EXISTS`rh_pe`.`attributes` (
`toid` INT(20) NOT NULL,
`Kommentar` VARCHAR(60) NOT NULL,
`Aktion` varchar(10) NOT NULL,
`Person1` INT NOT NULL,
`Person2` INT
) ENGINE = InnoDB;
The Foreign key should be like so:
ALTER TABLE `attributes`
ADD CONSTRAINT `Const`
FOREIGN KEY (`toid`) REFERENCES `pes`(`id`)
ON DELETE RESTRICT
ON UPDATE RESTRICT;
To create the foreign key, I tried the following two options:
IF NOT EXISTS(
ALTER TABLE `attributes`
ADD CONSTRAINT `Const`
FOREIGN KEY (`toid`) REFERENCES `pes`(`id`)
ON DELETE RESTRICT
ON UPDATE RESTRICT
);
and
ALTER TABLE `attributes`
ADD CONSTRAINT `Const`
FOREIGN KEY
IF NOT EXISTS (`toid`) REFERENCES `pes`(`id`)
ON DELETE RESTRICT
ON UPDATE RESTRICT
But none of them work.
Any Ideas on how I could create the constraint only if it does not exist?

Answer 1

Both of your table examples have the same name, so I suposed that your second table name is "pes" as you mention in your constraint examples. This one should work:
IF NOT EXISTS (SELECT * FROM sys.objects o WHERE o.object_id = object_id(N'`rh_pe`.`Const`') AND OBJECTPROPERTY(o.object_id, N'IsForeignKey') = 1)
BEGIN
ALTER TABLE `rh_pe`.`attributes` ADD CONSTRAINT `Const` FOREIGN KEY (`toid`) REFERENCES `rh_pe`.`pes`(`id`) ON DELETE RESTRICT ON UPDATE RESTRICT;
END
I haven't used the "if (not) exists" clausule for this but you can find a similar question here: If Foreign Key Not Exist Then Add Foreign Key Constraint(Or Drop a Foreign Key Constraint If Exist) without using Name?