Forms :: Autofill One Of The Field In Form

I want to Autofill one of the field in my form, by checking specific conditions in my database. Example of which is as below:

1. Date of Maintenance :
2. Loom No. :
3. Component Replaced :
4. Component Type :
5. Latest date for same component replacement :

I want to autofill the Sr. No. 5 in the form itself by checking database, and if Sr. No.2=Sr No.3=Sr. 4, then latest date should be autofill in Sr. No. 5

Say, A component was replaced in a Loom on 01/01/2013 & again after six month same component is replaced on 01/06/2013 so while filling up the form, In sr. no. 5 latest date 01/01/2013 of replacement of same component in same loom should be pulled from data base.