1 个表上的 T-SQL 多重连接

Question

我目前正在 MS SQL Server 2012 中处理复杂的 T-SQL 查询。我基本上检索项目的基本列表，其中包含 ProjectId 以及 StaffVersionId（员工可能在整个项目中发生变化，虽然我希望不会是这样 :P )。

CREATE TABLE #BasicProjects
    (
        ProjectId INT
        , StaffVersionId INT
    )

接下来我需要加入 ProjectData 表以获取项目的标题，并加入 Employee 表以获取员工的全名。

SELECT [P].ProjectId
    , [PD].Label AS Title
    , [E].Lastname + '' '' + Firstname AS Manager
    , [E].Lastname + '' '' + Firstname AS Contact
    FROM #BasicProjects [P]
    INNER JOIN [MySchema].[ProjectData] [PD] ON [PD].ProjectDataId = [P].ProjectDataId 
    INNER JOIN [MySchema].[Staff] [Y] ON [Y].StaffVersionId = [P].StaffVersionId AND [Y].StaffTypeId = 3 // Manager
    INNER JOIN [MySchema].[Staff] [X] ON [X].StaffVersionId = [P].StaffVersionId AND [X].StaffTypeId = 2 // Contact
    INNER JOIN [dbo].[Employee] [E] ON [E].EmployeeId = [Y].EmployeeId
    INNER JOIN [dbo].[Employee] [E] ON [E].EmployeeId = [X].EmployeeId

查询有点难，因为我有 2 种员工员工 3 = 经理，2 = 联系人。

当我运行我的查询时，我收到了这个错误：

在 FROM 子句中多次指定相关名称“E”。

你知道如何解决这个错误吗？或者关于如何改进这个查询的一些建议？

非常感谢！

Answer 1

您需要为每个连接使用不同的别名（就像您为 Staff 所做的那样）

SELECT [P].ProjectId
    , [PD].Label AS Title
    , [YE].Lastname + ' ' + YE.Firstname AS Manager
    , [XE].Lastname + ' ' + XE.Firstname AS Contact
    FROM #BasicProjects [P]
    INNER JOIN [MySchema].[ProjectData] [PD] ON [PD].ProjectDataId = [P].ProjectDataId 
    INNER JOIN [MySchema].[Staff] [Y] ON [Y].StaffVersionId = [P].StaffVersionId AND [Y].StaffTypeId = 3 // Manager
    INNER JOIN [MySchema].[Staff] [X] ON [X].StaffVersionId = [P].StaffVersionId AND [X].StaffTypeId = 2 // Contact
    INNER JOIN [dbo].[Employee] [YE] ON [YE].EmployeeId = [Y].EmployeeId
    INNER JOIN [dbo].[Employee] [XE] ON [XE].EmployeeId = [X].EmployeeId