mysql: add / editing a foreign key after database was created

This task is simple but offten needed.

If you have created a databasetable and you forgot the foreignkeys, you can simply add it later via altering table.

Add a foreignkey

	ALTER TABLE post
	ADD FOREIGN KEY (`userid`)
	REFERENCES user(`id`)
	ON DELETE CASCADE
	ON UPDATE CASCADE

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This key references the userid from users to a tupel in table posts. If a user gets deleted or updated the post will be deleted to0.

Deleting a foreign key

alter table post drop foreign key post_ibfk_1

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