Select top one from left outer join

问题

Guys, I have a query where basically select the latest browser that our user used.

here is our (simplified) table structure

HITS_TABLE
----------
USERID
BROWSER
HITSDATE

USER_TABLE
----------
USERID
USERNAME

and here is how I query the latest browser that our user used

SELECT U.*, H.BROWSER

FROM USER_TABLE U

CROSS APPLY 
  (SELECT TOP 1 BROWSER 
   FROM HITS_TABLE 
   WHERE HITS_TABLE.USERID = U.USERID
   ORDER BY HITS_TABLE.HITSDATE DESC
  )as H

The HITS_TABLE is just added several days ago.

So, that query is just resulting users that visited our website after we added the HITS_TABLE, and eliminate the others.

Here is the sample case

USER_TABLE
-------------------
USERID     USERNAME
-------------------
1          'Spolski'
2          'Atwoord
3          'Dixon'


HITS_TABLE
------------------------------
USERID     HITSDATE     BROWSER
------------------------------
2          15/8/2009    'Firefox 3.5'
1          16/8/2009    'IE 6'
2          16/8/2009    'Chrome'

Here is the sample result

------------------------------
USERID     USERNAME     BROWSER
------------------------------
1          'Spolsky'    'IE 6'
2          'Atwoord'    'Chrome'

But, I want to add other users with 'unknown' browser. Here is my desired result

------------------------------
USERID     USERNAME     BROWSER
------------------------------
1          'Spolsky'    'IE 6'
2          'Atwoord'    'Chrome'
3          'Dixon'      'Unknown'

I believe it could be achieved by LEFT OUTER JOIN. But I always had this: (I DO NOT want this result)

------------------------------
USERID     USERNAME     BROWSER
------------------------------
1          'Spolsky'    'IE 6'
2          'Atwoord'    'Chrome'
2          'Atwoord'    'Firefox 3.5'
3          'Dixon'      'Unknown'

I hope my question is clear.

回答1:

using a group by on userid against the hits_table allows you to get the max() hitsdate for each userid. I've called this LATEST HITS in the code below.

Selecting on the USER TABLE with a left join to LATEST HITS allows you to pull records for every user.

joining back onto the HITS TABLE then allwos you to pull the browser record associated with that date, or a null for users with no record in there.

select
   user_table.userid,
   user_table.username,
   isnull(hitstable.browser, 'unknown') as browser
from
  user_table
left join
(
  select
    userid,
    max(hitsdate) hitsdate
  from
    hits_table
  group by  
    userid
) latest_hits
on
  user_table.userid = latest_hits.userid    
left join
  hits_table
on hits.table.userid = latest_hits.userid
and hits_table.hitsdate = latest_hits.hitsdate

回答2:

Couldn't you sub select, not pretty but should work ..

SELECT U.*,

ISNULL((SELECT TOP 1 BROWSER 
   FROM HITS_TABLE 
   WHERE HITS_TABLE.USERID = U.USERID
   ORDER BY HITS_TABLE.HITSDATE DESC),'UnKnown') AS Browser

FROM USER_TABLE U

回答3:

SELECT U.*,'BROWSER' = 
    case 
     when (SELECT TOP 1 BROWSER FROM HITS_TABLE WHERE HITS_TABLE.USERID = U.USERID ORDER BY HITS_TABLE.HITSDATE DESC) is  null then 'Unknown'
else (SELECT TOP 1 BROWSER FROM HITS_TABLE WHERE HITS_TABLE.USERID = U.USERID ORDER BY HITS_TABLE.HITSDATE DESC)
    end
FROM USER_TABLE U

来源：https://stackoverflow.com/questions/1283787/select-top-one-from-left-outer-join

标签

sql-server-2005

outer-join