Hello

I got a problem. I got 2 fields in my Item form , 

1 Category and 2 sub category 

Q1. How to link a look up field to a field ,

I am importing my data and i got a coulmn call category , , i created a field call category and when i import data , i map that coulmn to category field, (which is empty at the min) i was assuming that when i import the data all the values of category should populate category lookup field with no duplicate . but its not happening , any suggeston 

Q2 how can i change the selection of sub category depending on the selection of category