Question

How to delete a child row (on delete cascade ?) when setting a null value on a parent?

Here’s the db design.

table A [id, b_id_1, b_id_2]

table B [id, other fields…]

b_id_1 and b_id_2 can be NULL

if any of them is null, it means NO B records for corresponding FK (there are 2 of them)

so (b_id_1,b_id_2) can be (null,null), (100, null), (null, 100_or_any_other_number) etc

How in one SQL query both set b_id_1 (or b_id_2) to null and delete all rows from B that have this id?

What FK design should be applied to the 2 tables?

THE QUERY SHOULD BE EXECUTED FROM THE PERSPECTIVE OF A TABLE!

Yes, if we use “On delete set null” and delete a record from B table it will work.

But the query MUST touch only A table !!

And you see, there’s not such a statement in mysql SQL syntax as “delete field value from a row”.

We can only set it to NULL.

That’s what I need.

Any ideas?

Answer 1

ON DELETE SET NULL

http://dev.mysql.com/doc/refman/5.1/en/innodb-foreign-key-constraints.html

[CONSTRAINT [symbol]] FOREIGN KEY
    [index_name] (index_col_name, ...)
    REFERENCES tbl_name (index_col_name,...)
    [ON DELETE reference_option]
    [ON UPDATE reference_option]

reference_option:
    RESTRICT | CASCADE | SET NULL | NO ACTION

UPDATE: If you want to do it from the perspective of A table (why on earth would you want that???), then you can’t get around a trigger, e. g.

DELIMITER ;;

CREATE TRIGGER tau_A AFTER UPDATE ON A
FOR EACH ROW    
BEGIN
    IF OLD.b_id_1 IS NOT NULL AND NEW.b_id_1 IS NULL;
    THEN
        DELETE FROM B WHERE id = OLD.b_id_1;
    END IF;
    IF OLD.b_id_2 IS NOT NULL AND NEW.b_id_2 IS NULL;
    THEN
        DELETE FROM B WHERE id = OLD.b_id_2;
    END IF;
END;;

Anyway, that won’t set to NULL any values in A that have the same value, because you cannot make a trigger in MySQL, that changes the same table that triggered the trigger.

And generally, if you want weird stuff like this, I can assert with high probability that your database design is flawed. If you put more details, I could perhaps suggest a better one.