SQL查询-连接同一列两次

Question

I'm having trouble to achieve the result I want trying join a column from a table twice.

My first table is "dbo.Sessions", which contains basic session info like the user ID, the project ID, login/logout date and times, etc.

I need to join to that the user names and project names. However, these are found in another table, but in the same column (dbo.tblObjects.Name).

Example:

+------+---------------+
| k_Id | Name          |
+------+---------------+
|   1  | AgentName1    |
|   2  | ProjectNameX  |
|   3  | ProjectNameY  |
|   4  | AgentName2    |
|   5  | ProjectNameZ  |
|   6  | AgentName3    |
+------+---------------+

To try and achieve my goal, I used two "LEFT JOIN". However, I get duplicate results in both. I'll either get both columns to display either the project names or the user names (depending on which "LEFT JOIN" is first).

This is what I have at this point:

SELECT SysDB.dbo.Sessions.*, SysDB.dbo.tblObjects.Name AS AgentName, SysDB.dbo.tblObjects.Name AS ProjectName
FROM SysDB.dbo.Sessions
LEFT JOIN SysDB.dbo.tblObjects ON SysDB.dbo.Sessions.userId = SysDB.dbo.Objects.k_Id
LEFT JOIN SysDB.dbo.tblObjects ON SysDB.dbo.Sessions.projectId = SysDB.dbo.Objects.k_Id
WHERE (SysDB.dbo.Sessions.loginDate BETWEEN 'm/d/yyyy' AND 'm/d/yyyy')

Note: SysDB is the name of the database that I identify every time because this query is to be run externally. I also don't use "USE SysDB" before my selection because it doesn't work from the VBA macro this will run from.

Note 2: I have found a thread on this site that addresses this exact issue, but I can't understand what is being done, and it dates back in 2012. Something about aliases. The solution offers to add "ls." and "lt." before the table names, but that doesn't work for me. Says the table doesn't exist.

SQL Query Join Same Column Twice

Note 3: I have tried many different things, such as:

LEFT JOIN SysDB.dbo.tblObjects AS AgentName ON SysDB.dbo.Sessions.userId = SysDB.dbo.tblObjects.k_Id
LEFT JOIN SysDB.dbo.tblObjects AS ProjectName ON SysDB.dbo.Sessions.projectId = SysDB.dbo.tblObjects.k_Id

Any insights would be greatly appreciated. Thanks!

Answer 1

You may find it much easier to see what you are doing by giving each table an alias ( session , agent , project below)

SELECT session.*, agent.Name AS AgentName, project.Name AS ProjectName
  FROM SysDB.dbo.Sessions session
       LEFT JOIN SysDB.dbo.tblObjects agent 
              ON session.userId = agent.k_Id
       LEFT JOIN SysDB.dbo.tblObjects project 
              ON project.projectId = session.k_Id
 WHERE (session.loginDate BETWEEN 'm/d/yyyy' AND 'm/d/yyyy')