MySQL updating all records having max value within group - mysql

Question

Apologies if this has been answered elsewhere and I'm just not seeing it; this is the closest I've found, but it isn't quite what I'm trying to do.
MySQL - updating all records to match max value in group
I have a table on a production web server with about 15,000 rows. There are many records sharing an item_name with another record, but item_meters is (normally, but not always) a unique value for each row. item_id is always unique. Currently every record has a value of "0" in the item_flag column.
I would like to update all records with the largest item_meters value within each item_name group to have an item_flag value of "1".
Here is a simplified version of the table ordered by item_id ASC:
----------------------------------------------
mytable
----------------------------------------------
item_id | item_name | item_meters | item_flag
--------+-----------+-------------+-----------
001 | aaa | 224 | 0
002 | aaa | 359 | 0
003 | aaa | 456 | 0
004 | bbb | 489 | 0
005 | bbb | 327 | 0
006 | bbb | 215 | 0
007 | ccc | 208 | 0
008 | ccc | 756 | 0
009 | ccc | 756 | 0
--------+-----------+-------------+-----------
The desired result would be a table with "1" in the item_flag column for each "aaa" having the largest item_meters, each "bbb" having the largest item_meters, each "ccc" having the largest item_meters, etc. like this:
----------------------------------------------
mytable
----------------------------------------------
item_id | item_name | item_meters | item_flag
--------+-----------+-------------+-----------
001 | aaa | 224 | 0
002 | aaa | 359 | 0
003 | aaa | 456 | 1
004 | bbb | 489 | 1
005 | bbb | 327 | 0
006 | bbb | 215 | 0
007 | ccc | 208 | 0
008 | ccc | 756 | 1
009 | ccc | 756 | 0
--------+-----------+-------------+-----------
(In case there are 2 or more records having the same item_name and the same item_meters (e.g. item_id 008 and 009 above), the desired result would be for the record with the numerically lower item_id (item_id is always unique), to have an item_flag value of "1" while the row with a numerically higher item_id would still have an item_flag value of "0")
Also of note, even though this database is running behind a production web server with new rows added every day, there will be no need to update the table every time a new row is added. It is something that will only be required once, regardless of whether new rows are later added outside of the parameters. The reason I mention this, is because execution speed is not a big concern since the query will only be executed once.
Thank you in advance! Please let me know if I can provide more info or clarify my question in any way.

Answer 1

The approach I take is to first write a query (SELECT statement) that will return the item_id values of the rows we want to update.
As a starting point, get the maximum value for item_meters, a simple query like this:
SELECT m.item_name
, MAX(m.item_meters) AS max_item_meters
FROM my_table m
GROUP BY m.item_name
We can use that query as an inline view in another query, to get the lowest item_id for each of those item_name
SELECT MIN(o.item_id) AS min_item_id
FROM ( SELECT m.item_name
, MAX(m.item_meters) AS max_item_meters
FROM my_table m
GROUP BY m.item_name
) n
JOIN my_table o
ON o.item_name = n.item_name
AND o.item_meters = n.max_item_meters
GROUP BY o.item_name, o.item_meters
And we can use that query as an inline view that gets the whole row associated with the item_id values we returned...
SELECT t.item_id
, t.item_name
, t.item_meters
, t.item_flag
FROM my_table t
JOIN ( SELECT p.min_item_id
FROM ( SELECT MIN(o.item_id) AS min_item_id
FROM ( SELECT m.item_name
, MAX(m.item_meters) AS max_item_meters
FROM my_table m
GROUP BY m.item_name
) n
JOIN my_table o
ON o.item_name = n.item_name
AND o.item_meters = n.max_item_meters
GROUP BY o.item_name, o.item_meters
) p
) q
ON q.min_item_id = t.item_id
Once the SELECT query is working, convert that to an UPDATE statement... replace the SELECT ... FROM with UPDATE, and add a SET clause. (Sometimes, it's necessary to wrap the inline view in yet another SELECT, to avoid MySQL error about disallowing references to the table we are updating.)
UPDATE my_table t
JOIN ( SELECT p.min_item_id
FROM ( SELECT MIN(o.item_id) AS min_item_id
FROM ( SELECT m.item_name
, MAX(m.item_meters) AS max_item_meters
FROM my_table m
GROUP BY m.item_name
) n
JOIN my_table o
ON o.item_name = n.item_name
AND o.item_meters = n.max_item_meters
GROUP BY o.item_name, o.item_meters
) p
) q
ON q.min_item_id = t.item_id
SET t.item_flag = '1'
If the intent is not to UPDATE the existing table, but to return a resultset, we can write a query and do an outer join to that same inline view, and return either a 0 or a 1 for item_flag, testing whether the item_id matches one we want flagged as 1...
SELECT t.item_id
, t.item_name
, t.item_meters
, IF(q.min_item_id IS NULL,0,1) AS `item_flag`
FROM my_table t
LEFT
JOIN ( SELECT p.min_item_id
FROM ( SELECT MIN(o.item_id) AS min_item_id
FROM ( SELECT m.item_name
, MAX(m.item_meters) AS max_item_meters
FROM my_table m
GROUP BY m.item_name
) n
JOIN my_table o
ON o.item_name = n.item_name
AND o.item_meters = n.max_item_meters
GROUP BY o.item_name, o.item_meters
) p
) q
ON q.min_item_id = t.item_id