Using the DLookup function, you can "look up" data in another field based on a choice the user makes in a list. Instead of including the value you want to display in a query with a join, you effectively run a separate query to fill the control each time it's displayed. Because each lookup expression runs its own query, this strategy is usually only appropriate if you're looking up one field. (For an example of this technique, see the "Combo 4" form in AC208.MDB.)
Any expression reference in one control's ControlSource property to another control in the same form causes automatic recalculation to occur. One control can have its ControlSource set to be an expression doing a DLookup based upon the value in another control. For example, to display an employee's phone extension in a text box when the employee's ID was selected using a multicolumn combo box, you would set the text box's ControlSource to something like:
=DLookup("[Extension]", "Employees", "[Employee Id] = " & [Combo Name])
Note that the example would be typed in as one line in a text box's ControlSource property.
Note Run performance comparisons using this method against joining the tables in a query. Depending on the amount of data one method may be quicker than another.