Joins here (in ABAP) are not those Native SQL Joins. If you are talking about the Core RDBMS, which mean Oracle or SQL Server, then Undoubtedly Joins are the best.
In ABAP, these joins are first split by the ABAP processor and then sent to the database, with the increase in DATA in production system, these joins tend to give way if your database keeps growing larger and larger.
You should rather used "FOR ALL ENTRIES IN" (Tabular conditions), which is a much effecient way as far as performance is concerned.
For example :
DATA: BEGIN OF LINE,
CARRID TYPE SPFLI-CARRID,
CONNID TYPE SPFLI-CONNID,
CITYFROM TYPE SPFLI-CITYFROM,
CITYTO TYPE SPFLI-CITYTO,
END OF LINE,
ITAB LIKE TABLE OF LINE.
LINE-CITYFROM = 'FRANKFURT'.
LINE-CITYTO = 'BERLIN'.
APPEND LINE TO ITAB.
LINE-CITYFROM = 'NEW YORK'.
LINE-CITYTO = 'SAN FRANCISCO'.
APPEND LINE TO ITAB.
SELECT CARRID CONNID CITYFROM CITYTO
INTO CORRESPONDING FIELDS OF LINE
FROM SPFLI
FOR ALL ENTRIES IN ITAB
WHERE CITYFROM = ITAB-CITYFROM AND CITYTO = ITAB-CITYTO.
WRITE: / LINE-CARRID, LINE-CONNID, LINE-CITYFROM, LINE-CITYTO.
ENDSELECT.
