Just to go with Paul's responses -
1. You should not be storing this data, if it is the same data (but only partial) from another field.
2. You just get at it when using a query. And, just so you know, you can use a query in 99.99% of the places you use a table, so you do NOT need to store this data IF you still have the other data stored. If you are trying to get rid of the first field and just have the other part, that is different.