MySQL selecting distinct value not working properly - mysql

Question

I am trying to retrieve distinct value using this query from my salary table
SELECT DISTINCT gross,sl_month,sl_year
FROM wp_attn_emp_salary where `emp_id` = 22 order by gross DESC
But it returns duplicate value too. please, check the image.
Query Result
My expecting result is like this
Array
(
[0] => stdClass Object
(
[gross] => 9595
[sl_month] => 2
[sl_year] => 2021
)
[1] => stdClass Object
(
[gross] => 9226
[sl_month] => 1
[sl_year] => 2020
)
[2] => stdClass Object
(
[gross] => 8875
[sl_month] => 2
[sl_year] => 2019
)
[3] => stdClass Object
(
[gross] => 8420
[sl_month] => 1
[sl_year] => 2019
)
[4] => stdClass Object
(
[gross] => 8405
[sl_month] => 12
[sl_year] => 2018
)
[5] => stdClass Object
(
[gross] => 6464
[sl_month] => 1
[sl_year] => 2018
)
)
I need the sl_month,sl_year in descending order too.

Answer 1

You misunderstand how DISTINCT works in SQL. It returns a row if the whole row (i.e. all columns) is distinct from any other row in the result. These two rows are distinct from each other:
gross
sl_month
sl_year
9595
1
2021
9595
2
2021
From the desired result you described, it looks like you want a greatest-n-per-group query.
SELECT gross, sl_month, sl_year
FROM (
SELECT gross, sl_month, sl_year,
ROW_NUMBER() OVER (PARTITION BY gross ORDER BY sl_year, sl_month) AS rownum
FROM wp_attn_emp_salary
WHERE emp_id = 22
) AS t
WHERE t.rownum = 1
ORDER BY sl_month DESC, sl_year DESC;
Note: Window functions require MYSQL 8.0, which was released in 2018 so you should be using it by now. If you need a solution that runs on an earlier version of MySQL, search past answers with the greatest-n-per-group tag.