Need to understand the Cascading of Drop down concept when data is from DB tables.

Hi,
Could someone help me understand the concept and solve my below problem?

I have a Two tables

1. VehicleTypes
Field
Id   category
1    Cars
2    Buses
3   Trains

2nd table SubVehicleTypes

SubID Id SubCategory
1         1     Sedan
2         1     Suv
3         2     Luxury
4         2     Semi luxury
5         3     Ac Trains
6         3     Non Ac Trains

Requirements:
1. Drop Down 1 will have data from Table 1
2. Drop Down 2 will have data from Table 2

When I select category from DD1 the second DD2 should give Subcategory based on ID(Common in both Tables).

Thanks
Faisal