How do I improve my union statement

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SQL

I am currently writing a sql script to return data if a user searches by certain criteria
*Supplier
*Approver
*Capturer

I have 2 tables which are joined
1 users/supplier

the users belong to a group which would determine if they are approver or capturer
Groupid Description
1 approver
2- Capturer

now the issue i am having is when trying filter data

so if the user wants to search for a supplier and an approver but limit the capturer i have a slight issue.my script is in the what have i tried below.

How could i improve my script

What I have tried:

Select Supplier.id,Supplier.SupplierDescription,User.Username Approver,''capturer
from supplier,user
where user.groupid = 1 --Approver
Union all 
Select Supplier.id,Supplier.SupplierDescription,''Approver,user.Username Capturer
from supplier,user
where user.groupid = 2 --Capturer

Now if the user decided they would like to search for all suppliers and 1 approver and all capturers

this is the script i currently have

Select Supplier.id,Supplier.SupplierDescription,User.Username Approver,''capturer
from supplier,user
where user.groupid = 1 --Approver
and user.Userkey = 150
Union all 
Select Supplier.id,Supplier.SupplierDescription,''Approver,user.Username Capturer
from supplier,user
where user.groupid = 2 --Capturer
and supplier.id in(Select Supplier.id
from supplier,user
where user.groupid = 1 --Approver
and user.Userkey = 150)