microsoft sql force 2 columns to have unique values
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I am tying to make a table for related products so products will have a many on many relation.  I want to make sure products van not have a relation with them self.

My table looks like this:

    ProductID int
    RelatedToProductID int
    Active bit
and has values like this:

    productID RelatedToProductID Active
    1         2                  1
    1         3                  1
    2         1                  1
    2         2                  1

but I want to exclude relations like the last one, were a products has a relation to itself.

Does anyone know how to make that possible in ms sql?