1. Table in design view, select the fields simultaneously, set as primary key
2. Simplest code might be a DLookup function. The real trick is figuring out what even to put code in. Maybe a 'Save Record' button click. How restrictive is the data? Can there be more than one unit per date per Site?
If Not IsNull(DLookup("ID", "tablename", "Unit='" & Me.Unit & "', AND [Date]=#" & Me.Date & "# AND [Time]='" & Me.Time & "' AND Site='" & Me.Site & "'")) Then
MsgBox "Record for this date, time, unit, site already exists."
Else
'code to save record, quit form
End If
Review this
http://office.microsoft.com/en-us/ac...010341717.aspx