An important concept to understand in Access is Nulls. A field's value can be null when it has not been assigned a value. I recently read an updated FAQA post on UtterAccess about nulls which opened my eyes to one aspect of working with nulls. So here's that article, plus some slightly more basic stuff that will be of more general interest
First some advice on working with null data in queries. This is important- many a novice user has got results different than he expected by ignoring the impact of nulls. See the following articles from Allen Browne:
This article at UtterAccess discusses some of the interesting aspects of working with nulls, especially with regards to concatentating strings. It also has links to a whole bunch of other articles on nulls.