how to read a colum upto say 10 rows and if we get a different value we have to stop

In my recent development i am getting a problem,i have 2 tables cdhdr and cdpos,i ve select data and read in i_cdhdr and i_cdpos resp.

Now i have to select changenr from cdhdr table till we get me28 tcode in tcode colun like if we ve say 10 me28 and then me23n and then me28,we ve 2 chose only first 10 me28. and then pass to cdpos table and get value_new and again pass the changenr to cdhdr to get user details,this is for po realease.