Left Join without duplicate rows from left table

Please look at the following query:

tbl_Contents

Content_Id  Content_Title    Content_Text
10002   New case Study   New case Study
10003   New case Study   New case Study
10004   New case Study   New case Study
10005   New case Study   New case Study
10006   New case Study   New case Study
10007   New case Study   New case Study
10008   New case Study   New case Study
10009   New case Study   New case Study
10010   SEO News Title   SEO News Text
10011   SEO News Title   SEO News Text
10012   Publish Contents SEO News Text

tbl_Media

Media_Id    Media_Title  Content_Id
1000    New case Study   10012
1001    SEO News Title   10010
1002    SEO News Title   10011
1003    Publish Contents 10012

QUERY

SELECT 
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

RESULT

10002   New case Study  2014-03-31 13:39:29.280 NULL
10003   New case Study  2014-03-31 14:23:06.727 NULL
10004   New case Study  2014-03-31 14:25:53.143 NULL
10005   New case Study  2014-03-31 14:26:06.993 NULL
10006   New case Study  2014-03-31 14:30:18.153 NULL
10007   New case Study  2014-03-31 14:30:42.513 NULL
10008   New case Study  2014-03-31 14:31:56.830 NULL
10009   New case Study  2014-03-31 14:35:18.040 NULL
10010   SEO News Title  2014-03-31 15:22:15.983 1001
10011   SEO News Title  2014-03-31 15:22:30.333 1002
10012   Publish         2014-03-31 15:25:11.753 1000
10012   Publish         2014-03-31 15:25:11.753 1003

10012 are coming twice...!

My query is returning duplicate rows from tbl_Contents (left table in the join)

Some rows in tbl_Contents has more than 1 associated rows in tbl_Media. I need all rows from tbl_Contents even if there are Null values exists in the tbl_Media BUT NO DUPLICATE RECORDS.

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.

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.

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

来源：https://stackoverflow.com/questions/22769641/left-join-without-duplicate-rows-from-left-table

标签

sql

join

duplicates