如何外部联接同一表的多个实例

Question

我有两个表，即tbl_Certificates和tbl_Employees。 证书由员工负责。 在某些情况下，prepareBy和approvdby属性可以为NULL，但是我仍然想要所有证书列表。 预期结果如上图所示。 我知道这是一个非常基本的问题。 提前致谢...：）

Answer 1

SELECT * FROM tbl_Certificates AS Certs
    LEFT JOIN tbl_Employees AS PreparedBy ON Certs.PrepareById=PreparedBy.Id
    LEFT JOIN tbl_Employees AS ApprovedBy ON Certs.ApprovedById=ApprovedBy.Id

Answer 2

简而言之：您只需这样做

select
    cert.id                 as "CertificateId"
    ,preparedBy.Name        as "PreparedBy"
    ,preparedBy.Designation as "Prepared By Designation"
    ,approvedBy.Name        as "ApprovedBy"
    ,approvedBy.Designation as "Approved By Designation"
from
    tbl_Certificates cert
    left outer join tbl_Employees preparedBy
        on preparedBy.id = cert.PreparedById
    left outer join tbl_Employees approvedBy
        on preparedBy.id = cert.ApprovedById