Re: [GENERAL] How to create unique index on multiple columns wherethe combination doesn't matter?

On 23 Mar 2017, at 3:56 PM, Alban Hertroys <haramrae@gmail.com> wrote:

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
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