与3个表相关的SQL COUNT个条目

Question

I had this sql statement running for these two tables: tblStations and tblThreads, for counting number of Stations in each thread:

SELECT tblThreads.*, (SELECT COUNT(*) FROM tblStations WHERE tblStations .fldThreadID=tblThreads.fldID) AS TotalStationsInThread FROM tblThreads;

and then display tblThread.Name and TotalStationsInThread, for each thread in the system.

Now I added another table (tblUsers) in this hierarchy: each thread can have many users, and each user can have many stations. The three tables are related to each other by this:

tblStations.fldUserID=tblUsers.fldID > tblUsers.fldThreadID=tblThreads.fldID.

So I changed my SQL query to this:

SELECT tblThreads.*, (SELECT COUNT(*) FROM tblStations WHERE tblStations.fldUserID=tblUsers.fldID AND tblUsers.fldThreadID=tblThreads.fldID) AS TotalStationsInThread FROM tblThreads;

But now I'm getting this message: "No value given for one or more required parameters." It's like the database can't connect the tables tblStations with tblThreads via tblUsers...

Any help please on how to count the number of stations that are connected to all the users that are connected to each thread??

Answer 1

This is the correct answer for MS Access Jet Database Engine:

SELECT tblThreads.*, (SELECT COUNT(tblStations.fldUserID) FROM tblStations INNER JOIN tblUsers ON tblStations.fldUserID = tblUsers.fldID WHERE  tblUsers.fldThreadID = tblThreads.fldID)  AS TotalStationsInThread FROM tblThreads;

Many thanks for Gordon Linoff for his answer.

Answer 2

You need to include tblUsers in the from clause:

SELECT tblThreads.*,
       (SELECT COUNT(*)
        FROM tblUsers inner join
             tblStations
             on tblStations.fldUserID = tblUsers.fldID
        WHERE tblUsers.fldThreadID = tblThreads.fldID
       ) AS TotalStationsInThread
FROM tblThreads;