How to implement an optional relation?

Question

I have a table A and B, both look like this:

CREATE TABLE A (id integer CONSTRAINT aid PRIMARY KEY);

Now i want to model an optional one-to-many relation between A and B. Normally, I would extend the definition of B like so:

CREATE TABLE B (
    id integer CONSTRAINT bid PRIMARY KEY, 
    a_id integer REFERENCES A(id)
);

but now a_id can't be null , so the relation isn't optional. So basically I want to be able to assign null to this column, but if the value is not null then it must reference A.id .

Answer 1

Yes, a_id can be null in table B . I tried it:

tmp=# CREATE TABLE A (id integer CONSTRAINT aid PRIMARY KEY);
CREATE TABLE
tmp=# CREATE TABLE B (
tmp(#     id integer CONSTRAINT bid PRIMARY KEY, 
tmp(#     a_id integer REFERENCES A(id)
tmp(# );
CREATE TABLE
tmp=# insert into B (id,a_id) VALUES (3,null);
INSERT 0 1
tmp=#

Your current data model works.

Answer 2

您创建了另一个与A和B相关的表，该表仅包含“可选”关系