How to Retrieve the Latest Information Using SQL's `GROUP BY` and `MAX` (or Alternatives)?

Retrieving the Latest Information Using SQL Group By & Max

In some scenarios, it becomes necessary to retrieve the most recent or maximum values grouped by specific columns in a dataset. One such common aggregation is to find the latest information for unique combinations of values.

Consider the following table containing a list of alarms:

id	name	alarmId	alarmUnit	alarmLevel
1	test	voltage	psu	warning
2	test	voltage	psu	ceasing
3	test	voltage	psu	warning
4	test	temp	rcc	warning
5	test	temp	rcc	ceasing

The objective is to retrieve only the most recent information for each combination of alarmId and alarmUnit. The expected result should appear as follows:

id	name	alarmId	alarmUnit	alarmLevel
3	test	voltage	psu	warning
5	test	temp	rcc	ceasing

Several approaches can be utilized to achieve this result. One common method involves the use of the ROW_NUMBER() function, which assigns a rank to each row within each group. The following query takes advantage of this function:

SELECT *
FROM (
    SELECT *, ROW_NUMBER() OVER (PARTITION BY alarmId, alarmUnit ORDER BY id DESC) AS rn
    FROM mytable
) q
WHERE rn = 1

This query returns the most recent row for each combination of alarmId and alarmUnit.

For databases that do not support the ROW_NUMBER() function, such as MySQL, a different approach can be employed:

SELECT mi.*
FROM (
    SELECT alarmId, alarmUnit, MAX(id) AS mid
    FROM mytable
    GROUP BY alarmId, alarmUnit
) mo
JOIN mytable mi
ON mi.id = mo.mid

This query utilizes a subquery to identify the maximum id for each combination of alarmId and alarmUnit, and then joins the result with the original table to retrieve the corresponding rows.

In PostgreSQL versions 8.3 and below, the DISTINCT ON clause can be utilized:

SELECT DISTINCT ON (alarmId, alarmUnit) *
FROM mytable
ORDER BY alarmId, alarmUnit, id DESC

This query returns the first distinct row for each combination of alarmId and alarmUnit, which effectively provides the latest information. By harnessing these techniques, developers can efficiently retrieve the most recent values from their data, enabling them to make informed decisions based on the latest available information.

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