>>>When creating a unique index and FK constraint on two columns, does the order of columns matter?
>>>
>>>For example, I want to have unique index on SITE_NO (Int) and CATEGORY (Char(20) and a FK Constraint on these two fields. Would [SITE_NO],[CATEGORY] create different "results" from [CATEGORY],[SITE_NO]?
>>>
>>>TIA
>>
>>Yes, order of columns matter in the index. Select most selective column first.
>
>What do you mean by "most selective"?

If you have many categories and just few sites, then using Category as first column in the index may help with the queries. If it's the opposite situation of column's selectivity, then the index should start with the SiteNo column.