Mysql sort by date and group by results differently - mysql

Question

My table structure as follows
id parent last_date sub_type
----------------------------------
11 9 2017-02-28 1101
10 9 2016-08-26 1101
8 1 2017-02-20 1101
12 12 2016-08-31 1102
14 12 2016-12-20 1102
9 9 2016-12-31 1101
13 12 2017-03-23 1102
2 1 2017-01-25 1101
1 1 2016-12-31 1101
i want to fetch rows for each sub_type based the date (longest first) . i tried following query
SELECT * FROM mytable GROUP BY sub_type ORDER BY ISNULL(last_date) DESC, last_date DESC
and it results
id parent last_date sub_type
--------------------------------
1 1 2016-12-31 1101
12 12 2016-08-31 1102
But i expect below result .
id parent last_date sub_type
--------------------------------
13 12 2017-03-23 1102
11 9 2017-02-28 1101
Please guide me to get above result .
EDIT:
last_date may have NULL value which will max precedence over dated entries. Thatswhy i choose ISNULL DESC order.

Answer 1

You can use a correlated subquery in the WHERE clause with ORDER BY and LIMIT 1 to find the id of the row you are looking for.
SELECT *
FROM mytable t1
WHERE id = (
SELECT id
FROM mytable t2
WHERE t2.sub_type = t1.sub_type
ORDER BY ISNULL(last_date) DESC, last_date DESC, id DESC
LIMIT 1
)
Demo: http://rextester.com/DSPH11524
Note: sub_type should be indexed.

Answer 2

You can also do it by giving a row number based on the sub_type column and in the descending order of the last_date column.
Query
select t1.`id`, t1.`parent`, t1.`last_date`, t1.`sub_type` from
(
select `id`, `parent`, `last_date`, `sub_type`,
(
case `sub_type` when #A
then #R := #R + 1
else #R := 1 and #A := `sub_type` end
) as `rn`
from `your_table_name` t,
(select #R := 0, #A := '') r
order by `sub_type`, `last_date` desc
)t1
where t1.`rn` = 1;
Sql Fiddle demo

Answer 3

This is a typical question fetching one record in each group by some aggregation. Try this:
select
mytable.*
from mytable
join (
select max(last_date) as last_date, sub_type from mytable group by sub_type
) t1 on mytable.sub_type = t1.sub_type and mytable.last_date = t1.last_date
See this article How to select the first/least/max row per group in SQL.
and Related link at right:
Retrieving the last record in each group.
And demo in sqlfiddle.
Edit:
if there is no 2 same last date and null precedence, then try this:
select
mytable.*
from mytable
join (
select
max(last_date) as last_date,
max(isnull(last_date)) nulled,
sub_type
from mytable
group by sub_type
) t1 on mytable.sub_type = t1.sub_type
and (t1.nulled and mytable.last_date is null or (t1.nulled <> 1 and mytable.last_date = t1.last_date))
also demo in sqlfiddle.

Answer 4

You have written a wrong query.
Specify Where condition after where clause
Below query will give your expected result.
SELECT id,parent,max(last_Date),sub_type FROM mytable GROUP BY sub_type