3 joins and where clause together - sql

Question

I have 3 tables
bl_main (bl_id UNIQUE, bl_area)
bl_details (bl_id UNIQUE, name)
bl_data(bl_id, month, paper_tons, bottles_tons)
bl_id is not unique in the last table. There will be multiple rows of same bl_id.
I am trying to retrieve data in the following way
bl_id | name | bl_area | sum(paper_tons) | sum (bottles_tons) | paper_tons | bottles_tons
sum(paper_tons) should return the sum of all the paper tons for the same bl_id like Jan to December.
Using the below query i am able to retrieve all the data correctly except in the result, there are multiple occurances of bl_ids(From bl_data table).
SELECT bl_main.bl_id,name,bl_area,sums.SummedPaper, sums.SummedBottles,paper_tons,bottles_tons
FROM bl_main
JOIN bl_details ON
bl_main.bl_id= bl_details.bl_id
left outer JOIN bl_data ON
bl_data.bl_id= bl_main.bl_id
left outer JOIN (
SELECT bl_id, SUM(Paper_tons) As SummedPaper, SUM(bottle_tons) As SummedBottles
FROM bl_data
GROUP by bl_id) sums ON sums.bl_id = bl_main.bl_id
I wanto retrieve only the unique values of bl_ids without repetition and it should contain the bl_id which has the max month and not all the months for the same bl_id.
For ex:
INCORRECT
**0601** University Hall 75.76 17051 1356 4040 1154 **11**
**0601** University Hall 75.76 17051 1356 9190 101 **12**
**0605** UIC Student 22.86 3331 14799 0 356 **8**
CORRECT
**0601** University Hall 75.76 17051 1356 9190 101 **12**
**0605** UIC Student 22.86 3331 14799 0 356 **8**
I know I can get the max value using
WHERE Month = (SELECT MAX(Month)
but where exactlt should i add this in the query and should i change the join definition.
Any help is highly appreciated as i am new to sql. Thanks in advance.

Answer 1

You have two tables that probably should be combined into one (bl_main and bl_details). But putting that aside, what you need is a self-join subquery to select the row with the max month. Something like the following (untested):
SELECT bl_main.bl_id, bl_details.name, bl_main.bl_area, sums.sum_paper_tons,
sums.sum_bottles_tons, maxmonth.paper_tons, maxmonth.bottles_tons
FROM bl_main
INNER JOIN bl_details ON bl_main.bl_id = bl_details.bl_id
LEFT OUTER JOIN (SELECT bl_id, SUM(paper_tons) AS sum_paper_tons,
SUM(bottles_tons) AS sum_bottles_tons
FROM bl_data
GROUP BY bl_id) sums ON bl_main.bl_id = sums.bl_id
LEFT OUTER JOIN (SELECT bl_id, paper_tons, bottles_tons
FROM bl_data data2
INNER JOIN (SELECT bl_id, MAX(month) AS max_month
FROM bl_data
GROUP BY bl_id) m
ON m.bl_id = data2.bl_id
AND m.max_month = data2.month) maxmonth
ON bl_main.bl_id = maxmonth.bl_id

Answer 2

You can join the table containing the month against itself, using a subquery of the form:
Select *
From mytable m
Inner Join (Select max(Month) as Month, myId
From mytable
Group By myId) mnth
On mnth.myId = m.myId and mnth.Month = m.Month

Answer 3

Your JOIN clause
left outer JOIN bl_data ON
bl_data.bl_id= bl_main.bl_id
does not specify which month to select for the data you are displaying with paper_tons and bottles_tons.
You could update that JOIN to only contain the max month, and this should limit the entries, like so:
left outer JOIN (SELECT bl_id, MAX(Month) as Month from bl_data GROUP BY bl_id) as Month
ON Month.bl_id = bl_main.bl_id
left outer JOIN bl_data ON
bl_data.bl_id = bl_main.bl_id AND bl_data.Month = Month.bl_Month

Answer 4

I think this query is what you are looking for
SELECT bl_main.bl_id,name, bl_area, sums.SummedPaper, sums.SummedBottles, paper_tons, bottles_tons
FROM bl_main
JOIN bl_details ON bl_main.bl_id= bl_details.bl_id
left outer JOIN bl_data ON bl_data.bl_id= bl_main.bl_id
left outer JOIN
(
SELECT bl_id, month, SUM(Paper_tons) As SummedPaper, SUM(bottle_tons) As SummedBottles
FROM bl_data
WHERE month in
(SELECT MAX(month) FROM bl_data GROUP BY bl_id)
GROUP BY bl_id, month
) sums ON sums.bl_id = bl_main.bl_id

Answer 5

I wanted to just add a comment to the answer lc gave, but I don't have 50 reputation points yet. It is a link to an article that I believe explains this question and adds the why the solution that lc gave is correct.
http://www.sqlteam.com/article/how-to-use-group-by-with-distinct-aggregates-and-derived-tables