Creating a column in dataset for later filter use (mysql)
Data should obly be posted as text or ddl, that makes the live easier for all of us
Basically you query can be reduced to an If clause, as seen below, but as i tested it again, i noticed, mysql doesn't like you date format so it has to be converted to a date, so that the code can properly work.
Schema (MySQL v5.6)
CREATE TABLE sales (
  `Date` VARCHAR(10),
  `Sales` INTEGER
);

INSERT INTO sales
  (`Date`, `Sales`)
VALUES
  ('14.05.2018', '8046'),
  ('11.02.2020', '4564'),
  ('25.07.2018', '6648'),
  ('25.08.2020', '7785'),
  ('01.09.2020', '8788'),
  ('30.04.2018', '4655'),
  ('22.09.2019', '6867'),
  ('09.12.2016', '6868'),
  ('26.10.2018', '6785'),
  ('01.09.2020', '7456'),
  ('12.12.2018', '7456');


Query #1
select `Date`, `Sales`,
IF(CURRENT_DATE() - INTERVAL 6 DAY &lt;= STR_TO_DATE(`Date`,'%d.%c.%Y')
            AND CURRENT_DATE() + INTERVAL 6 DAY &gt;=  STR_TO_DATE(`Date`,'%d.%c.%Y'),1,0) flag
from sales;

| Date       | Sales | flag |
| ---------- | ----- | ---- |
| 14.05.2018 | 8046  | 0    |
| 11.02.2020 | 4564  | 0    |
| 25.07.2018 | 6648  | 0    |
| 25.08.2020 | 7785  | 0    |
| 01.09.2020 | 8788  | 1    |
| 30.04.2018 | 4655  | 0    |
| 22.09.2019 | 6867  | 0    |
| 09.12.2016 | 6868  | 0    |
| 26.10.2018 | 6785  | 0    |
| 01.09.2020 | 7456  | 1    |
| 12.12.2018 | 7456  | 0    |


View on DB Fiddle