Question

I have a non-directed graph table in my SQL Server (2008) database and I want a constraint to prevent duplicates.
My link table has 2 columns, (City1, City2), both integer references into the Cities table. I can use the following:

ALTER TABLE dbo.CityConnections
ADD CONSTRAINT CK_CityConnections_OneWayOnly UNIQUE (City1, City2)

Which does half of what I want but this does not prevent adding a “reverse” connection.

Google showed me

... UNIQUE (MIN(City1,City2), MAX(City1,City2))

Which looks like it will do exactly what I want but the syntax is invalid .

I have also tried a complicated constraint using CASE but I couldn’t get this syntax correct either.

How can I achieve this?

Answer 1

Assuming that you can control your entry to the table, the following should work;

ALTER TABLE dbo.CityConnections
ADD CONSTRAINT CK_CityConnections_Unq UNIQUE (City1, City2)
ALTER TABLE dbo.CityConnections
ADD CONSTRAINT CK_CityConnections_OneWayOnly CHECK (City1 < City2)