Validate relations between 2 fields in a form

Hello Everyone

how can i create a form contain tow fields ,the tow field have a relation like next photos

1-seg 1 has sub segnamed sub seg 1

2-after select sub seg2 i will change the parent (Segment to seg2)

3-after it the Sub-seg become clear 

Although if i change it manually the Sub-seg will become empty

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when i created 2 tables like these and create the relation between the 2 tables

and added them to a form then created a lookup method like next photo

but all the department displayed without select the locations

so i need to display the departments depend on the location(The first choice)

Thank you