建表

create table numbers
(
Number int,
Frequency int 
);
insert into numbers values(0, 7);
insert into numbers values(1, 1);
insert into numbers values(2, 3);
insert into numbers values(3, 1);

开窗函数思路：

从后往前和从前往后的频数相加，两个数都需要大于等于总数一半，再取平均

select avg(number) median
from(
    select number,
           sum(Frequency) over (order by number) a, -- 从前往后 频数相加
           sum(Frequency) over (order by number desc) b, -- 从后往前 频数相加
           sum(Frequency) over () c -- 总数，开窗了就不用group by了
    from Numbers
) n1
where a >= c/2 and b >= c/2

变量做法

select avg(Number) as median from
(
 select Number, Frequency, @sum as sum1, @sum:=Frequency+@sum as sum2
 from Numbers,(select @sum:=0) t
 order by Number
) t
where if
(
@sum&1, sum1<=floor(@sum/2) and sum2>floor(@sum/2),
sum1<=(@sum/2) and sum2>=(@sum/2)
)

此题我觉得逻辑能解决的非要写到sql里就是很麻烦，我估计出题人的本意是可以用存储过程来处理