为什么 LEFT join 会像 INNER join 一样省略左侧的一行

Question

你能告诉我为什么在下面选择 LEFT join 作为 INNER join 工作吗？

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

提前致谢
埃利亚斯

Answer 1

以下是我在 SAP 论坛中对同一问题的回答。

问题和答案已经在这里、这里等很多地方给出了，但让我用我的话再解释一下：

• 这是因为您正在“左连接”表的列上的

  WHERE

  子句中进行过滤。
• 如果“左连接”表中没有行，则其列将被视为 NULL 值，

  WHERE

  条件将失败。
• 一般解决方案：将条件移至

  ON

  子句。

在ABAP SQL中，不接受在ON子句中包含复杂的函数（例如ABAP 7.58中的

= ( SELECT ... )

），替代方法是将其保留在

WHERE

子句中并指示

OR ... IS NULL

：

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

另一个测试，任何人都可以重现（

DE

存在于表

T005T

中，但不存在

ZY

）：

TYPES tt_land1 TYPE STANDARD TABLE OF t005-land1 WITH EMPTY KEY.
TYPES: BEGIN OF ts_country_code_and_name,
         land1 TYPE t005t-land1,
         landx TYPE t005t-landx,
       END OF ts_country_code_and_name.
TYPES tt_country_codes_and_names TYPE STANDARD TABLE OF ts_country_code_and_name WITH EMPTY KEY.

DATA(countries) = VALUE tt_land1( ( 'DE' ) ( 'ZY' ) ).
DATA(country_codes_and_names) = VALUE tt_country_codes_and_names( ).
SELECT
    FROM @countries AS countries
    LEFT JOIN t005t
     ON t005t~land1 = countries~table_line
    AND t005t~spras = 'E'
    FIELDS countries~table_line AS land1, t005t~landx
    WHERE ( t005t~LAND1 IS NULL
         OR t005t~LAND1 IN ( SELECT land1 from t005 WHERE land1 LIKE 'D%' ) )
    INTO TABLE @country_codes_and_names.
ASSERT country_codes_and_names = VALUE tt_country_codes_and_names(
    ( land1 = 'DE' landx = 'Germany' )
    ( land1 = 'ZY' landx = '' ) ).

Answer 2

在互联网上搜索后，我在这里找到了答案
左连接限制

所以我使用CTE如下

WITH
+invkind AS ( SELECT ebeln, MAX( invkind ) AS invkind FROM zkrh_scmt_invh GROUP BY ebeln )
SELECT FROM @it_selected_data AS selected
  LEFT JOIN +invkind AS invh
    ON invh~ebeln = selected~ebeln
  LEFT JOIN dd07t
    ON invh~invkind = dd07t~domvalue_l AND
       dd07t~domname = 'ZTEST' AND
       dd07t~ddlanguage = @sy-langu
FIELDS selected~*, concat_with_space( invh~invkind, dd07t~ddtext, 2 ) AS invkind
INTO CORRESPONDING FIELDS OF TABLE @it_final_data.

非常感谢
埃利亚斯