==========================================================================

SELECT SINGLE d~signatur INTO signatur               
         FROM zivhd AS a INNER JOIN zpivdt  AS b
                                 ON b~invoice_no = a~tag_no
                         INNER JOIN zt1_rdt AS c
                                 ON c~objectid   = b~po_no
                         INNER JOIN zt0_pwd AS d
                                 ON d~uname      = c~empno
                         WHERE a~invoice = p_in.         

=============================================================================

SELECT SINGLE b~po_no INTO po_no1                   
                      FROM zivhd AS a INNER JOIN zpivdt AS b
                      ON b~invoice_no = a~tag_no
                      WHERE a~invoice = p_in.         

SELECT SINGLE empno INTO empno1
                FROM zt1_rdt WHERE objectid = po_no1.

SELECT SINGLE signatur INTO signatur
                FROM zt0_pwd WHERE uname = empno1. 

===========================================================================

위 두가지 select 문자에 inner join를 사용했는데

두가지 소스중에 어느것이 더 효율적인 소스인지 알고 싶습니다.