How to ORDER BY in each GROUP BY - mysql

Question

i have data like this:
Name
Class
Pos
Nam
A
5
Hung
B
1
Tran
A
6
Hoang
A
1
Bao
B
4
so now i want to make query on this table to group all students by class, in each class, studen will be sorted descending by index.
My expected table be like:
Name
Class
Pos
Tran
A
6
Nam
A
5
Hoang
A
1
Bao
B
4
Hung
B
1
Here is my query
SELECT * FROM
(
SELECT *
from
(
SELECT 'Nam' as name, "A" as class, 5 as pos
UNION
SELECT 'Hung' as name, "B" as class, 1 as pos
UNION
SELECT 'Tran' as name, "A" as class, 6 as pos
UNION
SELECT 'Hoang' as name, "A" as class, 1 as pos
UNION
SELECT 'Bao' as name, "B" as class, 4 as pos
)t0
ORDER BY pos DESC
)t1
GROUP BY name, class
But the output look so different:
Name
Class
Pos
Nam
A
5
Hung
B
1
Tran
A
6
Hoang
A
1
Bao
B
4
if i filter only class A, it look like
Name
Class
Pos
Hoang
A
1
Tran
A
6
Nam
A
5
only correct for class B:
Name
Class
Pos
Bao
B
4
Hung
B
1
Look like the order in sub-query can not be kept after group by.
My finally query will look like:
SELECT class as Class, COLLECT_LIST(name) as Name FROM
(
SELECT *
from
(
SELECT 'Nam' as name, "A" as class, 5 as pos
UNION
SELECT 'Hung' as name, "B" as class, 1 as pos
UNION
SELECT 'Tran' as name, "A" as class, 6 as pos
UNION
SELECT 'Hoang' as name, "A" as class, 1 as pos
UNION
SELECT 'Bao' as name, "B" as class, 4 as pos
)t0
ORDER BY pos DESC
)t1
GROUP BY class
If GROUP BY can keep order base on pos in sub-query then COLLECT_LIST will produce expected result like this:
Class
Name
A
[Tran, Nam,Hoang]
B
[Bao,Hung]
How to group and order internal in each group?
Thanks

Answer 1

The ORDER BY, in the middle of your query is useless. It should be put after the GROUP BY.
SELECT * FROM
(
SELECT *
from
(
SELECT 'Nam' as name, 'A' as class, 5 as pos
UNION
SELECT 'Hung' as name, 'B' as class, 1 as pos
UNION
SELECT 'Tran' as name, 'A' as class, 6 as pos
UNION
SELECT 'Hoang' as name, 'A' as class, 1 as pos
UNION
SELECT 'Bao' as name, 'B' as class, 4 as pos
)t0
)t1
WHERE class='A'
GROUP BY name, class
ORDER BY pos DESC;
output:
+-------+-------+-----+
| name | class | pos |
+-------+-------+-----+
| Tran | A | 6 |
| Nam | A | 5 |
| Hoang | A | 1 |
+-------+-------+-----+
An explanation about ORDER BY can be found here: https://mariadb.com/kb/en/why-is-order-by-in-a-from-subquery-ignored/
A "table" (and subquery in the FROM clause too) is - according to the
SQL standard - an unordered set of rows. Rows in a table (or in a
subquery in the FROM clause) do not come in any specific order.
EDIT: If you have ONLY_FULL_GROUP_BY enabled (which is the default in MySQL8.0+), you will get an error like this:
ERROR 1055 (42000): Expression #3 of SELECT list is not in GROUP BY
clause and contains nonaggregated column 't1.pos' which is not
functionally dependent on columns in GROUP BY clause; this is
incompatible with sql_mode=only_full_group_by
This is because grouping is done on name and class so SQL does not know which value to use for pos in that group. Should it that the highest value? of the lowest value? (or some other value...).
I did not notice (not enough coffee...). A re-write with this problems solved looks like this:
SELECT
name,
class,
min(pos) as pos
FROM
(
SELECT 'Nam' as name, "A" as class, 5 as pos
UNION
SELECT 'Hung' as name, "B" as class, 1 as pos
UNION
SELECT 'Tran' as name, "A" as class, 6 as pos
UNION
SELECT 'Hoang' as name, "A" as class, 1 as pos
UNION
SELECT 'Bao' as name, "B" as class, 4 as pos
)t1
WHERE class="A"
GROUP BY name, class
ORDER BY pos DESC;

Answer 2

If I understand your requirement correctly, you just need a two level sort first by class, then by index:
SELECT Class, GROUP_CONCAT(Name ORDER BY Pos DESC) AS Name
FROM yourTable
GROUP BY Class
ORDER BY Class;