How to check data entered into a form is accurate with data in a table?

[Kenny24]

I have a form (Form A), where I require the user to enter a CustomerID which they have created using a separate form (Form B) and which the data is stored in (Table B).

How would I go about checking the CustomerID they enter into Form A is an exact match to one of the CustomerID's stored in Table B?

I guess I'm struggling with finding the code to cross-reference between sources of data.

Any help or advise would be greatly appreciated.

[NeoPa]

Instead of allowing them to type the data in, why not provide a ComboBox which is populated with the valid entries from the other table?

[Kenny24]

That's one option.

How would I ensure that CustomerID's that have already made choices over what products they wish to purchase (and therefore, have their details stored in a separate table) are not present in the combo box?

To rephrase, I only want CustomerID's that haven't made their purchase choices available for picking.

[NeoPa]

You simply ensure the ComboBox is populated by those items which fit your criteria.

Something like :

SELECT tC.CustomerID
     , tC.CustomerName
FROM   [tblCustomer] AS [tC]
       LEFT JOIN
       [tblPurchase] AS [tP]
  ON   tC.CustomerID = tP.CustomerID
WHERE  (tP.CustomerID Is Null)