Fetch the row which has the Max value for a column
Table:
UserId, Value, Date.
I want to get the UserId, Value for the max(Date) for each UserId. That is, the Value for each UserId that has the latest date. Is there a way to do this simply in SQL? (Preferably Oracle)
Update: Apologies for any ambiguity: I need to get ALL the UserIds. But for each UserId, only that row where that user has the latest date.
This will retrieve all rows for which the my_date column value is equal to the maximum value of my_date for that userid. This may retrieve multiple rows for the userid where the maximum date is on multiple rows.
select userid,
my_date,
...
from
(
select userid,
my_Date,
...
max(my_date) over (partition by userid) max_my_date
from users
)
where my_date = max_my_date
"Analytic functions rock"
Edit: With regard to the first comment ...
"using analytic queries and a self-join defeats the purpose of analytic queries"
There is no self-join in this code. There is instead a predicate placed on the result of the inline view that contains the analytic function -- a very different matter, and completely standard practice.
"The default window in Oracle is from the first row in the partition to the current one"
The windowing clause is only applicable in the presence of the order by clause. With no order by clause, no windowing clause is applied by default and none can be explicitly specified.
The code works.
I see many people use subqueries or else vendor-specific features to do this, but I often do this kind of query without subqueries in the following way. It uses plain, standard SQL so it should work in any brand of RDBMS.
SELECT t1.*
FROM mytable t1
LEFT OUTER JOIN mytable t2
ON (t1.UserId = t2.UserId AND t1."Date" < t2."Date")
WHERE t2.UserId IS NULL;
In other words: fetch the row from t1 where no other row exists with the same UserId and a greater Date.
(I put the identifier "Date" in delimiters because it's an SQL reserved word.)
In case if t1."Date" = t2."Date" , doubling appears. Usually tables has auto_inc(seq) key, eg id . To avoid doubling can be used follows:
SELECT t1.*
FROM mytable t1
LEFT OUTER JOIN mytable t2
ON t1.UserId = t2.UserId AND ((t1."Date" < t2."Date")
OR (t1."Date" = t2."Date" AND t1.id < t2.id))
WHERE t2.UserId IS NULL;
Re comment from @Farhan:
Here's a more detailed explanation:
An outer join attempts to join t1 with t2. By default, all results of t1 are returned, and if there is a match in t2, it is also returned. If there is no match in t2 for a given row of t1, then the query still returns the row of t1, and uses NULL as a placeholder for all of t2's columns. That's just how outer joins work in general.
The trick in this query is to design the join's matching condition such that t2 must match the same userid, and a greater date. The idea being if a row exists in t2 that has a greater date, then the row in t1 it's compared against can't be the greatest date for that userid. But if there is no match -- ie if no row exists in t2 with a greater date than the row in t1 -- we know that the row in t1 was the row with the greatest date for the given userid.
In those cases (when there's no match), the columns of t2 will be NULL -- even the columns specified in the join condition. So that's why we use WHERE t2.UserId IS NULL , because we're searching for the cases where no row was found with a greater date for the given userid.
SELECT userid, MAX(value) KEEP (DENSE_RANK FIRST ORDER BY date DESC)
FROM table
GROUP BY userid
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