Choose balanced records from a table due to a column value

Question

I want to select records from a table which has a column with value -1 and +1 The ratio of -1 values to +1 values is about %17

But, I want to select records with balanced rate of -1 and +1 values.

For example I want to have select result with about %30 with -1 value and %70 with +1 value.

For example I have the following data set:

A B -1
A C -1
C Y +1
C A +1
C B +1
B C -1
A D +1
A F +1
D F +1
C F +1

Then, for example with %50 balanced data, I want to choose 4 records from these with exactly 2 records with value -1 and 2 records with value +1.

May you please help me how can I do this with SQL?

Thank you

Answer 1

since you didn't state the rules required clearly i could only guess this is what you want.

declare 
    @percentage_1   int,
    @percentage_2   int,
    @recs           int,
    @rn             int,
    @rp             int


select  
    @percentage_1   = 50,
    @percentage_2   = 50,
    @recs           = 4

select  
    @rn     = @percentage_1 * @recs / 100,
    @rp     = @percentage_2 * @recs / 100

; with 
cte as
(
    select  *, r = row_number() over (partition by value order by col1, col2)
    from    your_table
)
select  *
from    cte
where   (value  = -1    and r   <= @rn)
or      (value  = +1    and r   <= @rp)
order by value, col1, col2