从多个表引用一个外键

Question

I'm trying to connect one foreign key from multiple table's primary keys. Like this :

enter image description here

I've tried :

ALTER TABLE Table1 ADD CONSTRAINT entity FOREIGN KEY (entity_id)
    REFERENCES MainTable(entitiy_id);

or

ALTER TABLE MainTable 
    ADD FOREIGN KEY (entity_id) REFERENCES Table1(id),Table2(id),Table3(id); 

What should I do? Thanks.

Answer 1

it is not really clear what you want to achieve here. The foreign key should go from a non-key column of the "child" table to the key column of the referenced table.

In your diagram, I would expect each one of Table, Table2 and Table3 to have a column "parent_id", and in every one of them you add a foreign key, which goes from the "parent_id" column to the key of the MainTable.

That said, the code is

ALTER TABLE Table1 ADD COLUMN parent_id INT NULL;

ALTER TABLE Table1 ADD CONSTRAINT fk_table1 FOREIGN KEY (parent_id) REFERENCES MainTable(uniqueId);