How to combinate the select options using 'when........(.....or......) '

Dears,

I used select statement like

select rnn
         pucustctycode
    into table gt_zgrrnnh
         from zgrrnnh
         where rnn           in s_rnn
         and   lrcdate       gt gv_update
         and   deleteind     eq space.

and

select rnn
         pucustctycode
  appending table gt_zgrrnnh
         from zgrrnnh
         where rnn           in s_rnn
         and   lrcdate       eq gv_update
         and   lrctime       gt gv_uptime
         and   deleteind     eq space.

in my program. and now i want to combinate them using just one select.

and the different select conditions is lrdate and lrtime. and the firest selections has no limit to lrtime, but just limit to lrdate.

Anyone could help me how to use select ............when ..........(...........or............)?

Thanks a lot.

Julie.