MS Access选择多个联接

Question

I'm trying to select by c# and odbc from the following tables.

LinkTab (FromDevID, FromPort, ToDevID, ToPort)
DevList (ID,DevName...)

The result should look like

FromDevName  | FromPort | ToDevName | ToDevPort

i tried already the following statement:

SELECT dev1.DevName, lt.FromPort, dev2.DevName, lt.ToPort 
FROM (LinkTab lt
INNER JOIN DevList dev1 ON lt.FromDevID = dev1.ID) 
INNER JOIN devList dev2 ON lt.ToDevID = dev2.ID

and I couldn't get all records. I guess there is a mistake at my join condition.

Answer 1

Presumably, you need left join :

SELECT dev1.DevName, lt.FromPort, dev2.DevName, lt.ToPort 
FROM (LinkTab lt LEFT JOIN 
      DevList dev1
      ON lt.FromDevID = dev1.ID
     ) LEFT JOIN
     devList dev2
     ON lt.ToDevID = dev2.ID;

The reason your query would not return all the rows is because the devises may not always match. If this is the case, then INNER JOIN will filter out the rows with unmatched devices. A LEFT JOIN will keep all the rows in the first table, assigning NULL for the columns from the second and third tables.