How to calculate median in AWS Redshift?

Question

Most databases have a built in function for calculating the median but I don't see anything for median in Amazon Redshift.

You could calculate the median using a combination of the nth_value() and count() analytic functions but that seems janky. I would be very surprised if an analytics db didn't have a built in method for computing median so I'm assuming I'm missing something.

http://docs.aws.amazon.com/redshift/latest/dg/r_Examples_of_NTH_WF.html http://docs.aws.amazon.com/redshift/latest/dg/c_Window_functions.html

Answer 1

And as of 2014-10-17, Redshift supports the MEDIAN window function:

# select min(median) from (select median(num) over () from temp);
 min 
-----
 4.0

Answer 2

Try the NTILE function.

You would divide your data into 2 ranked groups and pick the minimum value from the first group. That's because in datasets with an odd number of values, the first ntile will have 1 more value than the second. This approximation should work very well for large datasets.

create table temp (num smallint);
insert into temp values (1),(5),(10),(2),(4);

select num, ntile(2) over(order by num desc) from temp ;
 num | ntile 
-----+-------
  10 |     1
   5 |     1
   4 |     1
   2 |     2
   1 |     2

select min(num) as median from (select num, ntile(2) over(order by num desc) from temp) where ntile = 1;
 median 
--------
      4