Generating Code for Registration Number

if you have seperate table for class and branch

 and you want 3 character of each in student id

 

you can do like below  accordnig to your condition

 

insert into tblregistration (studentid) values (select LEFT(branch.branchname, 3)+'-'+LEFT(class.classname, 3)+(select (ISNULL(MAX(id),0)+1) from tblregistration) from tblbranch branch inner join tblclass class on class.branchid=tblbranch.id)