MySQL 8.0

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CREATE TABLE my_table (
  country    VARCHAR(32),
  value      FLOAT,
  date       date
);
INSERT INTO
  my_table
VALUES
('a', 1.4, '2010-02-03'),
('a', 1.4, '2010-02-04'),
('a', 1.3, '2010-02-05'),
('a', 1.4, '2010-02-06'),
('a', 1.2, '2010-02-07'),
('a', 1.4, '2010-02-08'),
('a', 1.5, '2010-02-09'),
('a', 1.7, '2010-02-10'),
('a', 1.4, '2010-02-11'),
('a', 1.6, '2010-02-12'),
('a', 1.4, '2010-02-13'),
('a', 1.5, '2010-02-14'),
('a', 1.3, '2010-02-15'),
('a', 1.2, '2010-02-16'),
('b', 1.3, '2010-02-03'),
('b', 1.3, '2010-02-04'),
('b', 1.4, '2010-02-05'),
('b', 1.6, '2010-02-06'),
('b', 1.9, '2010-02-07'),
('b', 1.3, '2010-02-08'),
('b', 1.3, '2010-02-09'),
('b', 1.2, '2010-02-10'),
('b', 1.3, '2010-02-11'),
('b', 1.5, '2010-02-12'),
('b', 1.3, '2010-02-13'),
('b', 1.3, '2010-02-14'),
('b', 1.5, '2010-02-15'),
('c', 1.4, '2010-02-03'),
('c', 1.4, '2010-02-04'),
('c', 1.3, '2010-02-05'),
('c', 1.4, '2010-02-06'),
('c', 1.2, '2010-02-07'),
('c', 1.4, '2010-02-08'),

Records: 69  Duplicates: 0  Warnings: 0


WITH
  sorted AS
(
  SELECT
    *,
    ROW_NUMBER() OVER (
      PARTITION BY country
          ORDER BY date     -- choose a different column name!
    ) - 1
      AS row_id,
    COUNT(*) OVER (
      PARTITION BY country
    ) - 1
      AS step
  FROM
    my_table
)
SELECT
  sorted.*
FROM
  sorted
WHERE
  mod( row_id             * 4, step)
  <
  mod((row_id + step - 1) * 4, step)
ORDER BY
  country, row_id


country
value
date
row_id
step


a
1.4
2010-02-03
0
13

a
1.2
2010-02-07
4
13

a
1.7
2010-02-10
7
13

a
1.4
2010-02-13
10
13

a
1.2
2010-02-16
13
13

b
1.3
2010-02-03
0
12

b
1.6
2010-02-06
3
12

b
1.3
2010-02-09
6
12

b
1.5
2010-02-12
9
12

b
1.5
2010-02-15
12
12

c
1.4
2010-02-03
0
11

c
1.4
2010-02-06
3
11

c
1.5
2010-02-09
6
11

c
1.6
2010-02-12
9
11

c
1.5
2010-02-14
11
11

d
1.3
2010-02-03
0
10

d
1.6
2010-02-06
3
10

d
1.3
2010-02-08
5
10

d
1.3
2010-02-11
8
10

d
1.3
2010-02-13
10
10

e
1.4
2010-02-03
0
9

e
1.4
2010-02-06
3
9

e
1.4
2010-02-08
5
9

e
1.7
2010-02-10
7
9

e
1.6
2010-02-12
9
9

f
1.3
2010-02-03
0
8

f
1.4
2010-02-05
2
8

f
1.9
2010-02-07
4
8

f
1.3
2010-02-09
6
8

f
1.3
2010-02-11
8
8

country	value	date	row_id	step
a	1.4	2010-02-03	0	13
a	1.2	2010-02-07	4	13
a	1.7	2010-02-10	7	13
a	1.4	2010-02-13	10	13
a	1.2	2010-02-16	13	13
b	1.3	2010-02-03	0	12
b	1.6	2010-02-06	3	12
b	1.3	2010-02-09	6	12
b	1.5	2010-02-12	9	12
b	1.5	2010-02-15	12	12
c	1.4	2010-02-03	0	11
c	1.4	2010-02-06	3	11
c	1.5	2010-02-09	6	11
c	1.6	2010-02-12	9	11
c	1.5	2010-02-14	11	11
d	1.3	2010-02-03	0	10
d	1.6	2010-02-06	3	10
d	1.3	2010-02-08	5	10
d	1.3	2010-02-11	8	10
d	1.3	2010-02-13	10	10
e	1.4	2010-02-03	0	9
e	1.4	2010-02-06	3	9
e	1.4	2010-02-08	5	9
e	1.7	2010-02-10	7	9
e	1.6	2010-02-12	9	9
f	1.3	2010-02-03	0	8
f	1.4	2010-02-05	2	8
f	1.9	2010-02-07	4	8
f	1.3	2010-02-09	6	8
f	1.3	2010-02-11	8	8