sql subquery statement with a join - sql

Question

i am not able to think on how to build an sql statement that fits my needs, if this is just not possibl eplease let me know.
i have a table like this(ill ommit the columns not making dificulties to me)
table: servicio
------------------------------------------------------------
id | swo | date | issueValue |
1 | 15-001 | 2015-01-29 01:52:59 | 2
1 | 15-002 | 2015-01-30 01:24:00 | 2
------------------------------------------------------------
table: comments
------------------------------------------------------------
id | swo | date | Area |
1 | 15-001 | 2015-01-29 01:52:59 | 2
1 | 15-002 | 2015-01-30 01:24:00 | 1
1 | 15-002 | 2015-01-30 01:50:00 | 3
------------------------------------------------------------
i want to select the rows in servicio but include the latest Area assigned to each swo. the result should be something like this.
------------------------------------------------------------
id | swo | date | Area |
1 | 15-001 | 2015-01-29 01:52:59 | 2
1 | 15-002 | 2015-01-30 01:24:00 | 3
------------------------------------------------------------
so how can i make the sql statement check for top (1) and return the area value in it?

Answer 1

In SQL Server you can do this with apply, a subquery, or row_number(). Here is the first method:
select s.*, c.area
from servicio s outer apply
(select top 1 c.*
from comments c
where c.swo = s.swo
order by c.date desc
) c;
With this method, you can pull out additional columns if you like.

Answer 2

This is solution with Rownumber and subquery. In subquery you are selecting all rows which match between tables. Creating Ascending order for Comment table date column, Partitioning for every SWO (Partitioning means it will reset counter on every different SWO). 1 = Last date for particular SWO. In final select you just put WHERE clause with Rownum = 1
SELECT swo ,date ,issueValue ,Area
FROM
(SELECT
SRV.swo
,SRV.date
,SRV.issueValue
,CMNT.Area
,ROW_NUMBER() OVER (PARTITION BY CMNT.swo ORDER BY CMNT.date DESC) AS Dsc_Rank
FROM servicio AS SRV
LEFT JOIN comments AS CMNT
ON (SRV.swo=CMNT.swo)
) AS Temp
WHERE Temp.Dsc_Rank = 1 /* Descending Date Rank 1 = Latest date from Comment table*/
Note: for example in Teradata you can omit sub query and use Qualify clause