Inner Join + select the most recent - sql

Question

I have been trying to do the bellow query but honestly it's driving me crazy.
I have 2 Tables on MS SQL CE 4.0
Table 1 Name: Items
ID
Item_Code
Logged_by
Description
ID | Item_Code | Logged_by | Description
1 | A | Pete | just an A
2 | B | Mary | Seams like a B
3 | C | Joe | Obviously this is a C
4 | D | Pete | This is another A
Table 2 Name: Item_Comments
ID
Item_Code
Comment
Date
ID | Item_Code | Comment | Date
1 | B | Done | 2014/08/08
2 | A | Nice A | 2014/08/08
3 | B | Send 1 More | 2014/08/09
4 | C | Done | 2014/08/10
5 | D | This is an A | 2014/08/10
6 | D | Opps Sorry | 2014/08/11
The wanted result: I'm looking to join the most recent comment from Item_Comments to the Items Table
ID | Item_Code | Logged_by | Description | Comment
1 | A | Pete | just an A | Nice A
2 | B | Mary | Seams like a B | Send 1 More
3 | C | Joe | Obviously this is a C | Done
4 | D | Pete | This is another A | Opps Sorry
I did this query but I'm getting all the information =( mixed.
SELECT *
FROM Items t1
JOIN
(SELECT Item_Code, Comment, MAX(date) as MyDate
FROM Item_Comments
Group By Item_Code, Comment, Date
) t2
ON Item_Code= Item_Code
ORDER BY t1.Item_Code;
Do you know any way to do this ?

Answer 1

Try:
select x.*, z.comment
from items x
join (select item_code, max(date) as latest_dt
from item_comments
group by item_code) y
on x.item_code = y.item_code
join item_comments z
on y.item_code = z.item_code
and y.latest_dt = z.date
Fiddle test: http://sqlfiddle.com/#!6/d387f/8/0
You were close with your query but in your inline view aliased as t2 you were grouping by comment, leaving the max function to not actually aggregate anything at all. In t2 you should have just selected item_code and max(date) and grouped only by item_code, then you can use that to join into item_comments (y and z in my query above).
This is a second way of doing this using a subquery, however I would stick to the above (a join w/ an inline view):
select i.*, c.comment
from items i
join item_comments c
on i.item_code = c.item_code
where c.date = (select max(x.date)
from item_comments x
where x.item_code = c.item_code)
order by i.id
Fiddle test: http://sqlfiddle.com/#!6/d387f/11/0

Answer 2

Note if you run this inside piece you get every single record:
SELECT Item_Code, Comment, MAX(date) as MyDate
FROM Item_Comments
Group By Item_Code, Comment, Date
You want only the most recent comment. Assuming this is SQL Server 2008 or earlier, this get's you the most recent date for each Item_Code:
SELECT Item_Code, MAX(date) as MyDate
FROM Item_Comments
Group By Item_Code
Now you need to join that back and look up the comment on that date:
SELECT C.*
FROM Item_Comments C
INNER JOIN
(SELECT Item_Code, MAX(date) as MyDate
FROM Item_Comments
Group By Item_Code
) t2
ON C.Item_Code= t2.Item_Code
AND C.date = t2.MyDate
Now you can use that to join back to your original table:
SELECT t1.*, LatestComment.*
FROM Items t1
INNER JOIN
(
SELECT C.*
FROM Item_Comments C
INNER JOIN
(SELECT Item_Code, MAX(date) as MyDate
FROM Item_Comments
Group By Item_Code
) t2
ON C.Item_Code= t2.Item_Code
AND C.date = t2.MyDate
) LatestComment
On LatestComment.Item_Code = t1.Item_Code
Depending on the actual database you are using, this can get much simpler. Thats why you need to tag your database and version.

Answer 3

Try this,
create table items (id int, item_code char(1), logged_by varchar(10), description varchar(30));
insert into items values (1, 'A', 'Pete', 'just an A');
insert into items values (2, 'B', 'Mary', 'Seams like a B');
insert into items values (3, 'C', 'Joe', 'Obviously this is a C');
insert into items values (4, 'D', 'Pete', 'This is another A');
create table item_comments (id int, item_code char(1), comment varchar(20), date date);
insert into item_comments values (1, 'B', 'Done', '2014/08/08');
insert into item_comments values (2, 'A', 'Nice A', '2014/08/08');
insert into item_comments values (3, 'B', 'Send 1 More', '2014/08/09');
insert into item_comments values (4, 'C', 'Done', '2014/08/10');
insert into item_comments values (5, 'D', 'This is an A', '2014/08/10');
insert into item_comments values (6, 'D', 'Opps Sorry', '2014/08/11');
select * from items;
select * from item_comments;
select * from (select i.logged_by,i.id,i.item_code,i.description,ic.comment
,row_number() over(partition by i.id order by i.id )as Rnk
from items i inner join item_comments ic
on i.item_code=ic.item_code and i.id in(1,3)) x
where x.Rnk=1
union
select * from (select i.logged_by,i.id,i.item_code,i.description,ic.comment
,row_number() over(partition by i.id order by i.id )as Rnk
from items i inner join item_comments ic
on i.item_code=ic.item_code and i.id in(2,4)
) x where x.Rnk=2 order by item_code