android - 将RIGHT外部联接转换为左侧外部联接

我需要为sqlite设计一个查询
最初的查询是这样的

 FROM            PhysicianActivity INNER JOIN
                     Activity ON PhysicianActivity.ActivityID = Activity.ID RIGHT OUTER JOIN
                     PhysicianFrequency ON PhysicianActivity.PhysicianID = PhysicianFrequency.PhysicianID AND Activity.LegalEntityID = PhysicianFrequency.LegalEntityID

知道sqlite不支持右外部连接，所以我只能使用左外部连接，但无法将此查询转换为左外部连接
有人能帮忙吗？

最佳答案

大多数人喜欢LEFT OUTER JOIN。在大多数情况下，以下内容是等效的：

FROM  PhysicianFrequency LEFT JOIN
      PhysicianActivity
      ON PhysicianActivity.PhysicianID = PhysicianFrequency.PhysicianID LEFT JOIN
      Activity
      ON PhysicianActivity.ActivityID = Activity.ID AND
         Activity.LegalEntityID = PhysicianFrequency.LegalEntityID

我建议您使用表别名，以便查询更易于编写和读取：

FROM  PhysicianFrequency pf LEFT JOIN
      PhysicianActivity pa
      ON pa.PhysicianID = pf.PhysicianID LEFT JOIN
      Activity a
      ON pa.ActivityID = a.ID AND
         a.LegalEntityID = pf.LegalEntityID