Yes, the order doesn't matter, and this approach sounds like a good idea. I'll try it out, thanks.
On 22 Mar 2017, at 17:54, Glen Huang <hey.hgl@gmail.com> wrote:
Hello,
If I have a table like
CREATE TABLE relationship (
obj1 INTEGER NOT NULL REFERENCES object,
obj2 INTEGER NOT NULL REFERENCES object,
obj3 INTEGER NOT NULL REFERENCES object,
...
)
And I want to constrain that if 1,2,3 is already in the table, rows like 1,3,2 or 2,1,3 shouldn't be allowed.
Is there a general solution to this problem?
Does the order of the values of (obj1, obj2, obj3) in relationship matter? If not, you could swap them around on INSERT/UPDATE to be in sorted order. I'd probably go with a BEFORE INSERT OR UPDATE trigger.In addition, to prevent unsorted entry, on obj2 add CHECK (obj2 > obj1) and on obj3 add CHECK (obj3 > obj2).Now you can create a normal PK or unique key on (obj1, obj2, obj3) as the order of their values is not variable anymore.Alban Hertroys--If you can't see the forest for the trees,cut the trees and you'll find there is no forest.