>hi everyone,
> i'm a new user of this site. i don't know if my question was
> posted already. my problem is how can i check if the value entered is already
> existed on the database or if the value entered is null, etc.
> it's all about database/record validation.
>
> i used AERROR(), but unfortunately it didn't. perhaps i missed some
> codes.
>
> thanks for the help.
>
>_ayen01710

One way to check for uniqueness is with primary or candidate indices.

Even if you use them, you may still want to intercept the VFP error message, to show your own message. You can use the following function to check whether the uniqueness is violated, according to a certain index.

HTH,

Hilmar.

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