SQL Server bit column constraint, 1 row = 1, all others 0

Question

I have a bit IsDefault column. Only one row of data within the table may have this bit column set to 1, all the others must be 0.

Answer 1

You could apply an Instead of Insert trigger and check the value as it's coming in.

Create Trigger TRG_MyTrigger
on MyTable
Instead of Insert
as
Begin

  --Check to see if the row is marked as active....
  If Exists(Select * from inserted where IsDefault= 1)
  Begin
     Update Table Set IsDefault=0 where ID= (select ID from inserted);

     insert into Table(Columns)
     select Columns from inserted
  End

End

Alternatively you could apply a unique constraint on the column.