MySQL 5.7

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create table test (num int);
insert into test values (1);
insert into test values (2);
insert into test values (3);
insert into test values (4);
insert into test values (5);
insert into test values (6);
insert into test values (7);
Select *
from
(
    SELECT test.*, @counter := @counter +1 counter
    FROM (select @counter:=0) initvar, test
    ORDER BY num
) X
where counter <= round(50/100 * @counter);
ORDER BY num;


num
counter


1
1

2
2

3
3

4
4



You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'ORDER BY num' at line 1


Select *
from
(
    SELECT test.*, @counter := @counter +1 counter
    FROM (select @counter:=0) initvar, test
    ORDER BY num DESC
) X
where counter <= (50/100 * @counter);
ORDER BY num;


num
counter


7
1

6
2

5
3



You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'ORDER BY num' at line 1