sql - 有人可以帮我解决 SQL 中的这个连接语句吗？有两个 JOINS 相继

Question

在下图中 ([2]) 的第 20 行，您会看到“LEFT JOIN HUB_EMPLOYEES HM”，并且在加入 LINK_EMPLOYEES 后立即出现。

剩下的 HUB_EMPLOYEES 加入了什么？ LINK_EMPLOYEES 上的连接在第 21 行指定，但我看不到 HUB_EMPLOYEES 上的左连接如何工作。

如果有人能解释一下，我将不胜感激，我在别处找不到答案。

这是代码:

SELECT HE.EMPLOYEE_SEQNR, HE.EMPLOYEEID, SE.LASTNAME AS EMPLASTNAME,
       isnull(SM.LASTNAME, 'DIRECTOR') AS MNGLASTNAME, 
       SE.TITLE, SE.COUNTRY, SE.REGION, SE.CITY, SE.POSTALCODE 
FROM SAT_EMPLOYEES SE 
JOIN      HUB_EMPLOYEES HE ON SE.EMPLOYEE_SEQNR = HE.EMPLOYEE_SEQNR
LEFT JOIN HUB_EMPLOYEES HM
JOIN      LINK_EMPLOYEES ON HM.EMPLOYEE_SEQNR = LINK_EMPLOYEES.MANAGER_SEQNR
JOIN      SAT_EMPLOYEES SM ON HM.EMPLOYEE_SEQNR = SM.EMPLOYEE_SEQNR
ON HE.EMPLOYEE_SEQNR = LINK_EMPLOYEES.EMPLOYEE_SEQNR

Answer 1

相当于:

SELECT HE.EMPLOYEE_SEQNR, HE.EMPLOYEEID, SE.LASTNAME AS EMPLASTNAME,
       isnull(SubQ.LASTNAME, 'DIRECTOR') AS MNGLASTNAME, 
       SE.TITLE, SE.COUNTRY, SE.REGION, SE.CITY, SE.POSTALCODE 
FROM    SAT_EMPLOYEES SE 
JOIN      HUB_EMPLOYEES HE ON SE.EMPLOYEE_SEQNR = HE.EMPLOYEE_SEQNR
LEFT JOIN (
                SELECT  LINK_EMPLOYEES.EMPLOYEE_SEQNR
                ,       SM.LASTNAME
                FROM    HUB_EMPLOYEES HM
                JOIN      LINK_EMPLOYEES ON HM.EMPLOYEE_SEQNR = LINK_EMPLOYEES.MANAGER_SEQNR
                JOIN      SAT_EMPLOYEES SM ON HM.EMPLOYEE_SEQNR = SM.EMPLOYEE_SEQNR
        ) SubQ
ON HE.EMPLOYEE_SEQNR = SubQ.EMPLOYEE_SEQNR

我个人不喜欢这种语法，因为它不是很直观。