The JOIN
addition on SELECT
allows you to merge several data tables together into a single result or result table (mostly). As database servers are quite clever bits of machinery, using a join is not only nice coding, it can be very helpful in a good performance too.
A simple example:
SELECT hdr~vbeln itm~posnr
itm~matnr FROM vbuk AS hdr INNER JOIN vbup AS itm ON hdr~vbeln = itm~vbeln INTO TABLE gt_results WHERE hdr~vbeln in so_vbeln.