MariaDB 10.3

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CREATE TABLE stat (
  `line_name` INTEGER,
  `device_name` INTEGER,
  `count` INTEGER
);

INSERT INTO stat
  (`line_name`, `device_name`, `count`)
VALUES
  ('1001', '3548001', '2'),
  ('1002', '3548002', '3'),
  ('1003', '3548003', '6'),
  ('2001', '3548004', '7'),
  ('2002', '3548005', '3'),
  ('2003', '3548006', '4'),
  ('3001', '3548007', '3'),
  ('3002', '3548008', '9'),
  ('3003', '3548009', '7');
WITH cte AS 
(SELECT *, ROW_NUMBER() OVER (PARTITION BY LEFT(line_name, 1) ORDER BY RAND()) AS rn
 FROM stat)
SELECT `line_name`, `device_name`, `count`
FROM cte
WHERE rn = 1
ORDER BY RAND()
LIMIT 2


line_name
device_name
count


1003
3548003
6

3003
3548009
7


WITH cte AS 
(SELECT *, ROW_NUMBER() OVER (PARTITION BY LEFT(line_name, 1) ORDER BY RAND()) AS rn
 FROM stat)
SELECT `line_name`, `device_name`, `count`
FROM cte
WHERE rn = 1
ORDER BY RAND()
LIMIT 2


line_name
device_name
count


1001
3548001
2

3002
3548008
9


WITH cte AS 
(SELECT *, ROW_NUMBER() OVER (PARTITION BY LEFT(line_name, 1) ORDER BY RAND()) AS rn
 FROM stat)
SELECT `line_name`, `device_name`, `count`
FROM cte
WHERE rn = 1
ORDER BY RAND()
LIMIT 2


line_name
device_name
count


1002
3548002
3

2003
3548006
4