Thanks. Didn't realize it could be implemented with a exclusion constraint. The comparing between any two row definitely sounds like the right direction. But I'm still having a hard time figuring out how i should write the `exclude_element WITH operator` part, which I think, should detect if specified columns consist of the same items, regardless the order? could `exclude_element` contains multiple columns? (from the syntax it looks like it's impossible) And is there such an operator to compare multiple columns?
> On 23 Mar 2017, at 1:04 AM, David G. Johnston <david.g.johns...@gmail.com> > wrote: > > On Wed, Mar 22, 2017 at 9:54 AM, Glen Huang <hey....@gmail.com > <mailto:hey....@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? > > Sorry if the question is too basic, but I couldn't find the answer in the > doc, at least not in the chapter on unique index. > > The most direct option to consider is a exclusion constraint. > > https://www.postgresql.org/docs/current/static/ddl-constraints.html > <https://www.postgresql.org/docs/current/static/ddl-constraints.html> (bottom > of page) > > David J.
