Where clause in Joins

Question

I have a selection screen like:
Table       Field
EKKO     EKORG
EKKO     EKGRP
EKKO     LIFNR
ESSR     EBELN
ESSR     EBELP
ESSR     LBLNE
ESSR     FRGZU
ESSR     ERDAT
ESSR     LOEKZ
Due to further complexities in the program i am joining 3 tables EKKO, EKPO and ESSR together. I can write the select stmt in 2 ways. I want to understand which one is better in terms of performance and is there exactly a rule here that i should follow before i write the where clause in this join.
  SELECT aebeln bebelp alifnr aekorg aekgrp bpackno c~lblni
         clblne cerdat cpackno cloekz cfrgsx cfrgkl c~frgzu
         INTO  TABLE gt_test
         FROM  ekko AS a INNER JOIN ekpo AS b
                         ON  aebeln  = bebeln
                         INNER JOIN essr AS c
                         ON  aebeln = cebeln
                         AND bebeln = cebeln
                         AND bebelp = cebelp
         WHERE a~lifnr IN so_lifnr
           AND a~ekorg IN so_ekorg
           AND a~ekgrp IN so_ekgrp
           AND c~lblne IN so_lblne
           AND c~ebeln IN so_ebeln
           AND c~ebelp IN so_ebelp
           AND c~erdat IN so_erdat.
  SELECT aebeln bebelp alifnr aekorg aekgrp bpackno c~lblni
         clblne cerdat cpackno cloekz cfrgsx cfrgkl c~frgzu
         INTO  TABLE gt_test
         FROM  ekko AS a INNER JOIN ekpo AS b
                         ON  aebeln  = bebeln
                         INNER JOIN essr AS c
                         ON  aebeln = cebeln
                         AND bebeln = cebeln
                         AND bebelp = cebelp
         WHERE a~ebeln IN so_ebeln
           AND a~lifnr IN so_lifnr
           AND a~ekorg IN so_ekorg
           AND a~ekgrp IN so_ekgrp
           AND b~ebelp IN so_ebelp
           AND c~lblne IN so_lblne
           AND c~erdat IN so_erdat.
           AND c~erdat IN so_erdat.

Answer 1

Hi,
This part is not necessary:
... a~ebeln = c~ebeln ...
Modify:
AND c~ebeln IN so_ebeln
AND c~ebelp IN so_ebelp
by
AND a~ebeln IN so_ebeln
AND b~ebelp IN so_ebelp
The best option will be:
if not so_ebeln[] is initial.
   select ... where a~ebeln IN so_ebeln ... (2nd select)
else.
   select ... where a~lifnr IN so_lifnr ... (1st select)
endif.
Also, you can try only the 2nd select, because it has WHERE clause to ebeln and lifnr. Depending on your database, you can check (via ST04) if the right index is selected (eg. if you inform so_ebeln the database must do the selection using ebeln index. if you inform so_lifnr the database must do the selection using lifnr index, etc..)
Best regards,
Leandro Mengue