>Hilmar,
>
>I actually started using a unique integer table-driven primary key for my tables not too long ago (thanks Vlad!). What's the best way to ensure uniqueness in my candidate keys?
>
>Renoir

At the database level, uniqueness is enforced with candidate indices. Use them by all means. However, additionally, you may want to intercept before VFP does, to give the user more meaningful error messages than "Trigger Failed" or "Key violation".

For this purpose, use the following function before saving the record, and show the appropriate error message, like "SSN already exists":

**********************************************************************
FUNCTION DuplicateKey(tcTag, tcTable, txPkExpression)
	* Check for duplicate, according to a certain index tag.

	* Parameters:
	* tcTag: name of index tag.
	* tcTable (optional): name of table. Required if working with views.
	* txPkExpression (optional): primary key expression. Required when working with views.
	*   This allows to omit current record from checking of duplicate.
	*   Can't be used to check uniqueness of primary key (but I use non-business-value keys,
	*   automatically generated, anyway).

	* Returns: true if another record exists with the same value(s)
	* (according to the specified index tag).

	* Additional comments:

	* By intercepting for this duplicate value before Visual FoxPro does (through Primary or
	* Candidate Indices), the user can messages that are clearer than "Trigger failed"
	* (at least with Visual Extend, "Trigger failed" doesn't give more details).

	* The function can also be used to check duplicates in FoxPro 2.x;
	* just change "local" to "private".

	* Of course, depending on the index, this function may check whether the combination of
	* two or more fields is unique - however, only the index tag is passed as a parameter.

	local lnSelect, lcDbf, lnRecno, txEvalExpr, llDuplicate, lcExpression, lcOrder, llWorkOnView
	llWorkOnView = not empty(tcTable)
	lnSelect = select()
	lcDbf = iif(empty(tcTable), dbf(), tcTable)
	lnRecno = recno()
	lcOrder = order()
	if not llWorkOnView
		set order to (tcTag)
		lcExpression = key()
	else
		select 0
		use (lcDbf) order (tcTag) again
		lcExpression = key()
		use
		select (lnSelect)
	endif
	txEvalExpr = eval(lcExpression)
	select 0
	use (lcDbf) order (tcTag) again
	seek txEvalExpr
	if found() and (llWorkOnView and eval(tag(GetPkNum())) = txPkExpression;
			or not llWorkOnView and recno() = lnRecno)
		skip
	endif
	llDuplicate = (eval(lcExpression) = txEvalExpr)
	use
	select (lnSelect)
	set order to (lcOrder)
	return llDuplicate
ENDFUNC && DuplicateKey