What is not Possible with the chain set while doing migration?

How to display how many records are associate with the parent record when the both objects are in lookup relationship?

I have two objects one is a standard object i.e Account and the other one is a custom object and their have a look up relations .I have  created 3 text fields as A,B,C in  custom object.Now when I concatenate b/w A and B , the result should get displayed in C.when u type the value manually in field C ,it should throw the error.So which type of trigger is used and how wil u write the code

I have two objects one is a standard object i.e Account and the other one is a custom object and their have a look up relations .I have  created 3 text fields as A,B,C in  custom object.Now when I concatenate b/w A and B , the result should get displayed in C.when u type the value manually in field C ,it should throw the error.So which type of trigger is used and how wil u write the code