Joins and Aggregate Functions - sql

Question

I have 3 tables I need to join together. Once I join the first two, I'm going to have two columns, let's call them A and B.
The relationship between A and B is many-to-many. So we can have:
A B
1 1
1 2
2 1
2 3
Then, I need to join with a third table on the B column, giving me:
A B C
1 1 5
1 2 6
2 1 9
2 3 2
Now for my final result I only want one row for each unique A value, and I want to select that row based upon the MAX C value across that given A.
So in this example the final value would be:
A B C
1 2 6
2 1 9
I have the following query which works as expected, but I am fairly certain it is not the best way of doing it:
SELECT
Temp.A,
Temp.B,
Temp.C1
FROM
(SELECT DISTINCT
T1.A,
T2.B,
MAX(T3.C) OVER(PARTITION BY T1.A) AS C1
FROM T1
INNER JOIN T2 ON T1.X = T2.X
INNER JOIN T3 ON T2.B = T3.B) Temp
INNER JOIN T3 ON T3.B = Temp.B
WHERE Temp.C1 = T3.C

Answer 1

You query can be simplified:
You don't need select distinct in the subquery.
You don't need to join back to T3.
You can select the C value in the subquery.
Here is the revision:
SELECT Temp.A, Temp.B, Temp.C
FROM (SELECT T1.A, T2.B, T3.C, MAX(T3.C) OVER (PARTITION BY T1.A) AS C1
FROM T1 INNER JOIN
T2
ON T1.X = T2.X INNER JOIN
T3
ON T2.B = T3.B
) Temp
WHERE Temp.C1 = Temp.C
Do note that if T3 has duplicate maximum values, then this will return duplicates. To get just one, you can use row_number() instead:
SELECT Temp.A, Temp.B, Temp.C
FROM (SELECT T1.A, T2.B, T3.C,
ROW_NUMBER() OVER (PARTITION BY T1.A ORDER BY T3.C DESC) AS seqnum
FROM T1 INNER JOIN
T2
ON T1.X = T2.X INNER JOIN
T3
ON T2.B = T3.B
) Temp
WHERE seqnum = 1;