Joining 3 Tables into one result set - sql-server

Question

I am trying to join only select columns from 3 different tables and haven't been successful.
Table 1, Patient, has the following columns I need:
ExternalID, UserDefinedXML, ServiceSiteUid
Table 2, PDI, has the following columns I need:
Patient ID, FirstName, LastName, State
Table 3, ListServiceSite has the following columns I need:
ServiceSiteUid, Name
I need to join Patient and PDI based on the common ID columns ExternalID and PatientID, and then join Patient and ListServiceSite by ServiceSiteUid.
Here's what I have that has been unsuccessful:
SELECT
*
FROM
(SELECT
ExternalID, UserDefinedXml, ServiceSiteUid
FROM
Patient) A
INNER JOIN
(SELECT
[Patient ID], FirstName, LastName, State
FROM
PatDemogImport) B ON A.ExternalID = B.[Patient ID]
WHERE
UserDefinedXml IS NOT NULL;
I am very new to SQL so please have patience! Thank you in advance.

Answer 1

This should do it
SELECT *
FROM Patient as p
JOIN PatDemogImport as pdi
ON p.ExternalID = pdi.[Patient ID]
JOIN ListServiceSite as lss
ON lss.ServiceSiteUid = p.ServiceSiteUid
WHERE p.UserDefinedXml IS NOT NULL;

Answer 2

SELECT * FROM PATIENT
JOIN
PDI on PATIENT.ExternalID = PDI.ExternalID AND PATIENT.PatientID = PDI.PatientID
JOIN
ListServiceSite on ListServiceSite.ServiceSiteUid = PATIENT.ServiceSiteUid
where
UserDefinedXml IS NOT NULL;
That should give you what you're looking for. There's no need for subselects in this instance.
Essentially what you are doing is joining the tables based on the relationship between columns. So in the above we join PDI and PATIENT based on their 2 shared columns and ListServiceSite and PATIENT on ServiceSiteUid.
Please note that when you do a SELECT *, you will get duplicate columns in the result (since you're basically getting all the columns across all those tables) so I'd recommend specifying what you select. i.e. SELECT PATIENT.ExternalID, ....
Finally, I'd recommend reading up on the different kind of joins. The above is just a regular join but there are several others like OUTER JOIN INNER JOIN RIGHT JOIN LEFT JOIN and they all have differences.

Answer 3

SELECT Patient.*
FROM Patient INNER JOIN PatDemogImport as pdi ON
Patient.ExternalID=pdi.Patientid
Inner join ListServiceSite on
Patient.ServiceSiteUid=ListServiceSite.ServiceSiteUid
WHERE Patient.UserDefinedXml IS NOT NULL

Answer 4

Isn't it just as simple as this? You want an inner join and you just need to specify the columns you need. This should give you exactly the answer you're looking for. These other answers have the same idea, but they assume that the tables contain only the columns listed and no other columns. PatientID should be the same as ExternalID so no need to specify that column and C.ServiceSiteUid should be the same as in Patient.
SELECT A.ExternalID, A.UserDefinedXML, A.ServiceSiteUid,
B.FirstName, B.LastName, B.State,
C.Name
FROM Patient A
INNER JOIN PatDemogImport B ON B.PatientID = A.ExternalID
INNER JOIN ListServiceSite C ON C.ServiceSiteUid = A.ServiceSiteUid
WHERE A.UserDefinedXML IS NOT NULL