Question

I have a one to many relationship between two tables in mysql.

One called type with a primary of url

url
varchar(255)

And one called order with an index on url.

url          | id_no
varchar(255) | int

The order table can have many values in it and there is already a foreign key set up so that if a url is deleted in the table type it deletes all associated keys with a matching url in order.

Orders can also be deleted manually meaning that a url could exist in type that doesn’t have a relationship to the table order.

Is there a way to set up a foreign key so that if a url in type does not have a relationship to the table order then it will delete itself from type?

Looking at the mysql manual I cannot figure it out, I am confused by CASCADE as it says it will delete all associated keys, but I only want to delete a url from the table type if and only if there is no relationship between the two?

Thanks for your help!

Answer 1

You can easily do that with a DELETE trigger on the order table: Let it check, if there exist other rows with the same url, and if not delete it from type.

Another way to do it is creating a maintenance script similar to

DELETE FROM `type` WHERE url NOT IN (
  SELECT DISTINCT `url` FROM `order`
)

and run it on a regular or on-demand basis.