Question

In Sql Server 2005, I have a table with two integer columns, call them Id1 and Id2. I need them to be unique with in the table (easy enough with a unique index that spans both columns). I also need them to be unique in the table if the values are transposed between the two columns.

For example, SELECT * FROM MyTable returns

Id1   Id2 --------- 2     4 5     8 7     2 4     2  <--- values transposed from the first row 

How do I make a constraint that would prevent the last row from being entered into the table because they are the transposed values from the first row?

Answer 1

Create a check constraint that is bound to a user defined function that performs a select on the table to check for the transposed value.

Create table mytable(id1 int, id2 int) go  create Function dbo.fx_Transposed(@id1 int, @id2 int) returns bit as  Begin     Declare @Ret bit     Set @ret = 0     if exists(Select 1 from MyTable          Where id2 = @id1 and id1 = @id2)     Set @ret = 1     Return @ret End GO Alter table mytable add CONSTRAINT [CHK_TRANSPOSE] CHECK   (([dbo].[fx_Transposed]([ID1],[ID2])=(0))) GO Insert into mytable (id1, id2) values (1,2)  Insert into mytable (id1, id2) values (2,1)