MySQL 5.6

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

create table your_table (
id int auto_increment primary key, 
`Group` varchar(30))
insert into your_table (`Group`) values
('a'), ('a'), ('a'), ('a'),
('b'), ('b'), ('b'), ('b'),
('c'), ('c'), ('c'), ('c'), 
('d'), ('d'), ('d'), ('d')

Records: 16  Duplicates: 0  Warnings: 0


SELECT *
FROM your_table AS t
WHERE `Group` IN (
  SELECT `Group` FROM (
    SELECT `Group`
      FROM your_table
      GROUP BY `Group`
      ORDER BY RAND()
      LIMIT 2) q);


id
Group


13
d

14
d

15
d

16
d

5
b

6
b

7
b

8
b


SELECT t.*
FROM your_table t
JOIN (
  SELECT DISTINCT `Group`
  FROM your_table
  ORDER BY RAND()
  LIMIT 2
) g 
ON g.`Group` = t.`Group`;


id
Group


1
a

2
a

3
a

4
a

5
b

6
b

7
b

8
b