Problem:

When should you use an update rule when Migrating and using a Query?

Resolution:

The Update Rule is needed whenever you have one of the following radio buttons checked within the "Query Options" "If Table Already Exists" group:

Update No Insert

Update Insert

Delete Match

In these cases, you have to specify a key in the "Query Update Rules" dialog.  The key is used to match the rows from the query to the data in the local table.  The "Update Rule" column in the "Query Update Rules" dialog specifies what action you want to take on the columns that are not key columns.  You can do one of the following:

Replace

Add

Subtract

None

These actions only take place for "Update No Insert" or "Update Insert".  If you specify None, the field is left alone.  Replace means replace the records value with the value from the query.  Add and Subtract will perform those operations on numeric columns.