Form - Data validation to stop duplicate bookings Access - ms-access

Question

I am having problems coming up with a solution to the following problem:
I have a booking form, but when the user adds details I want to check my database to see if the exact vehicle has been booked out for those dates already. If it has, an error message would appear.

Answer 1

Your vehicles should have ID Numbers. So, when a user tries to book a vehicle, you check the ID against the table of booked vehicles. Something like this (aircode) should work:
Dim db as Database
Dim rec as Recordset
Set db = CurrentDB
Set rec = db.OpenRecordset ("SELECT CarID FROM tblBookedCars WHERE CarID = " & Me.cboCarsToBook.Selected & "")
If rec.EOF Then
'Your car isn't booked, so process your booking
Else
Msgbox "That car is already booked"
End If

Answer 2

Assuming that booked out cars have a start and end date, then a booking collision occurs when:
RequestStartDate <= EndDate
and
RequestEndDate >= StartDate
The above is thus a rather simply query, but if any collision occurs, the above will return records, and you simply don’t allow the booking. And this means you don’t have to maintain some huge “messy” table of cars and each date that the car is book. You ONLY need simple row attached to a given vehicle with the start and end date (and thus it easy to change dates - you only update the one booking reocrd.
Air code to build such a query will look much like this:
dim strWhere as string
dim dtRequeestStartDate as date
dim dtRequestEndDate as date
dim lngCarID as long
dtRequestStartDate = inputbox("Enter start Date")
dtRequestEndDate = inputbox("Enter end date")
lngCarID = inputbox("What car id")
strWhere="#" & format(dtRequestStartDate,"mm/­dd/yyyy") & "# <= EndDate" & _
" and #" & format(dtRequestEndDate,"mm/dd­/yyyy") & "# >= StartDate and CarID = " & lngCarID
if dcount("*","tableBooking",strW­here) > 0 then
msgbox "sorry, you cant book
...bla bla bla....
The above is just an example, and I am sure you would build a nice form that prompts the user for the booking dates. However, what is nice here is that the above simple query will return ANY collision.