为何LEFT JOIN表现如INNER JOIN？附ABAP查询代码求助

你的LEFT JOIN表现得像INNER JOIN，核心原因是WHERE子句中对右表（zkrh_scmt_invh）字段的过滤条件，会直接排除左表中没有匹配右表的行。
当左表@it_selected_data的某行在右表中找不到对应的ebeln时，invh~invkind的值为NULL，而NULL = (子查询结果)的判断永远不成立，导致这些行被WHERE条件过滤掉，最终结果就和INNER JOIN一致了。

有两种常用的修正方式：

方式一：将过滤条件移至LEFT JOIN的ON子句中 #

把原本在WHERE里的子查询条件放到关联条件里，这样只会影响右表的匹配逻辑，不会过滤左表无匹配的行：

SELECT FROM @it_selected_data AS selected
      LEFT JOIN zkrh_scmt_invh AS invh
        ON invh~ebeln = selected~ebeln
       AND invh~invkind = ( SELECT MAX( invkind ) FROM zkrh_scmt_invh WHERE ebeln = invh~ebeln )
    FIELDS selected~*, invh~invkind
    INTO CORRESPONDING FIELDS OF TABLE @it_final_data.

方式二：先预查询右表的最大invkind数据，再做关联 #

先把每个ebeln对应的最大invkind单独查出来，再和左表做LEFT JOIN，逻辑更清晰：

DATA: lt_max_invk TYPE TABLE OF zkrh_scmt_invh.

SELECT ebeln, MAX( invkind ) AS invkind
  FROM zkrh_scmt_invh
 GROUP BY ebeln
  INTO CORRESPONDING FIELDS OF TABLE @lt_max_invk.

SELECT FROM @it_selected_data AS selected
      LEFT JOIN @lt_max_invk AS max_inv
        ON max_inv~ebeln = selected~ebeln
    FIELDS selected~*, max_inv~invkind
    INTO CORRESPONDING FIELDS OF TABLE @it_final_data.

内容的提问来源于stack exchange，提问作者ekekakos