select and update rownum sender jdbc

Dear all

i am using PI 7.31 sender jdbc adapter.

table(TB_AA)  has 10000 row with pi_stat = 'R'   and then  sender jdbc adapter tried  10 row  in using rownum  every  120 second and update same rownum

but  result of select query and udpate query  were different.  not same record update for selecting data.

co_cd,  memid  is PK.

select co_cd , mem_id,  reg_n  FROM TB_AA  where PI_STAT = 'R' AND rownum <= 10

update  TB_AA  set  PI_STAT = 'S', where  PI_STAT = 'R' and rownum <= 10

query is wrong?  ,  what is correct update query with several PK ?