Question

I’m trying to remove an association table from my database. It contains two columns (a_id and b_id) referencing the tables a and b.

This table is unnecessary because in fact it’s an OneToMany relation. So I added column a_id in table b.

My problem: How can I transfer the existing entrys from assoc_a_b to b.a_id?

SELECT DISTINCT b.id, a.id FROM table_a AS a
JOIN assoc_a_b AS assoc ON a.id = assoc.a_id
JOIN table_b AS b ON b.id = assoc.b_id;

This select statement works. Can it be combined with an UPDATE statement? The UPDATE statement would look someway like:

UPDATE b SET a_id = a.id WHERE id = b.id;

using a.id and b.id from the select statement above.

Answer 1

update b set a_id = (SELECT assoc_a_b.a_id from assoc_a_b where assoc_a_b.b_id = b.id)