Does SQL have a way to group rows without squashing the group into a single row? - group-by

Question

I want to do a single query that outputs an array of arrays of table rows. Think along the lines of <table><rowgroup><tr><tr><tr><rowgroup><tr><tr>. Is SQL capable of this? (specifically, as implemented in MariaDB, though migration to AWS RDS might occur one day)
The GROUP BY statement alone does not do this, it creates one row per group.
Here's an example of what I'm thinking of…
SELECT * FROM memes;
+------------+----------+
| file_name | file_ext |
+------------+----------+
| kittens | jpeg |
| puppies | gif |
| cats | jpeg |
| doggos | mp4 |
| horses | gif |
| chickens | gif |
| ducks | jpeg |
+------------+----------+
SELECT * FROM memes GROUP BY file_ext WITHOUT COLLAPSING GROUPS;
+------------+----------+
| file_name | file_ext |
+------------+----------+
| kittens | jpeg |
| cats | jpeg |
| ducks | jpeg |
+------------+----------+
| puppies | gif |
| horses | gif |
| chickens | gif |
+------------+----------+
| doggos | mp4 |
+------------+----------+
I've been using MySQL for ~20 years and have not come across this functionality before but maybe I've just been looking in the wrong place ¯\_(ツ)_/¯

Answer 1

I haven't seen an array rendering such as the one you want, but you can simulate it with multiple GROUP BY / GROUP_CONCAT() clauses.
For example:
select concat('[', group_concat(g), ']') as a
from (
select concat('[', group_concat(file_name), ']') as g
from memes
group by file_ext
) x
Result:
a
---------------------------------------------------------
[[puppies,horses,chickens],[kittens,cats,ducks],[doggos]]
See running example at DB Fiddle.
You can tweak the delimiters such as ,, [, and ].

Answer 2

SELECT ... ORDER BY file_ext will come close to your second output.
Using GROUP BY ... WITH ROLLUP would let you do subtotals under each group, which is not what you wanted either, but it would give you extra lines where you want the breaks.