inner join 与 left join

今天写一个SD的程序，出了点小问题

原来的SQL语句如下

  SELECT k~vbeln k~auart   p~posnr k~audat p~matnr k~kunnr p~kwmeng
     p~netpr p~kpein p~netwr k~waerk k~stwae d~bstkd
    INTO TABLE gt_vbak
    FROM vbak AS k
    JOIN vbap AS p ON p~vbeln = k~vbeln
    JOIN vbkd AS d ON d~vbeln = p~vbeln AND d~posnr = p~posnr
    WHERE k~vtweg IN s_vtweg
     AND   k~audat IN s_audat
     AND   k~kunnr IN s_kunnr
     AND   k~auart IN s_auart.

但是数据库表VBKD和VBAP不一定是一一对应的

如有一张订单，在VBKD中有客户PO号码，但是对应的没有行项目号，这样写SQL语句就查找不出来，于是改用 LEFT JOIN，如下

  SELECT k~vbeln k~auart   p~posnr k~audat p~matnr k~kunnr p~kwmeng
     p~netpr p~kpein p~netwr k~waerk k~stwae d~bstkd
    INTO TABLE gt_vbak
    FROM vbak AS k
    JOIN vbap AS p ON p~vbeln = k~vbeln
*     JOIN vbkd AS d ON d~vbeln = p~vbeln AND d~posnr = p~posnr
    left JOIN vbkd AS d ON d~vbeln = p~vbeln AND d~posnr = p~posnr
    WHERE k~vtweg IN s_vtweg
     AND   k~audat IN s_audat
     AND   k~kunnr IN s_kunnr
     AND   k~auart IN s_auart.

即可

另外inner join与join应该是一样的