How to Find the Maximum Value for Each Distinct User ID in an Oracle Table?

Retrieving the Maximum Value for Each Unique User ID in Oracle

This guide demonstrates how to efficiently extract the maximum value associated with each distinct user ID from an Oracle table. Assume a table structured as follows:

<code>UserId, Value, Date</code>

The objective is to retrieve the UserId and its corresponding Value where the Date is the most recent for each user.

Here's an effective Oracle SQL query to achieve this:

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;

This query uses a LEFT OUTER JOIN to compare each row in mytable (aliased as t1) with all other rows (aliased as t2) having the same UserId. The join condition t1."Date" < t2."Date" ensures that only rows with a later date are considered.

The WHERE t2.UserId IS NULL clause filters the results. Rows where t2.UserId is NULL indicate that no other row with the same UserId and a later Date exists. Therefore, these rows represent the maximum date for each user.

Example:

If mytable contains the following data:

UserId	Value	Date
1	5	2023-03-08
1	10	2023-03-15
2	3	2023-04-01
2	7	2023-04-10

The query will return:

UserId	Value	Date
1	10	2023-03-15
2	7	2023-04-10

This approach is efficient and avoids the need for subqueries or window functions, making it a robust solution for Oracle databases.

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