Left Join without duplicate rows from left table

Question

Please look at the following query:

tbl_Contents

Content_Id  Content_Title    Content_Text
10002   New case Study   New case Study
1         


        

Answer 1

Try an OUTER APPLY

SELECT 
    C.Content_ID,
    C.Content_Title,
    C.Content_DatePublished,
    M.Media_Id
FROM 
    tbl_Contents C
    OUTER APPLY
    (
        SELECT TOP 1 *
        FROM tbl_Media M 
        WHERE M.Content_Id = C.Content_Id 
    ) m
ORDER BY 
    C.Content_DatePublished ASC

Alternatively, you could GROUP BY the results

SELECT 
    C.Content_ID,
    C.Content_Title,
    C.Content_DatePublished,
    M.Media_Id
FROM 
    tbl_Contents C
    LEFT OUTER JOIN tbl_Media M ON M.Content_Id = C.Content_Id 
GROUP BY
    C.Content_ID,
    C.Content_Title,
    C.Content_DatePublished,
    M.Media_Id
ORDER BY
    C.Content_DatePublished ASC

The OUTER APPLY selects a single row (or none) that matches each row from the left table.

The GROUP BY performs the entire join, but then collapses the final result rows on the provided columns.

Answer 2

You can do this using generic SQL with group by:

SELECT C.Content_ID, C.Content_Title, MAX(M.Media_Id)
FROM tbl_Contents C LEFT JOIN
     tbl_Media M
     ON M.Content_Id = C.Content_Id 
GROUP BY C.Content_ID, C.Content_Title
ORDER BY MAX(C.Content_DatePublished) ASC;

Or with a correlated subquery:

SELECT C.Content_ID, C.Contt_Title,
       (SELECT M.Media_Id
        FROM tbl_Media M
        WHERE M.Content_Id = C.Content_Id
        ORDER BY M.MEDIA_ID DESC
        LIMIT 1
       ) as Media_Id
FROM tbl_Contents C 
ORDER BY C.Content_DatePublished ASC;

Of course, the syntax for limit 1 varies between databases. Could be top. Or rownum = 1. Or fetch first 1 rows. Or something like that.

Answer 3

Using the DISTINCT flag will remove duplicate rows.

SELECT DISTINCT
C.Content_ID,
C.Content_Title,
M.Media_Id

FROM tbl_Contents C
LEFT JOIN tbl_Media M ON M.Content_Id = C.Content_Id 
ORDER BY C.Content_DatePublished ASC