MySQL Math - Is it possible to calculate a correlation in a query?

Question

In a MySQL (5.1) database table there is data that represents:

• how long a user takes to perform a task and
• how many items the user handled during the tas

Answer 1

Here's a rough implementation of the sample correlation coefficient as described in:

Wikipedia - Correlation and Dependence

create table sample( x float not null, y float not null );
insert into sample values (1, 10), (2, 4), (3, 5), (6,17);

select @ax := avg(x), 
       @ay := avg(y), 
       @div := (stddev_samp(x) * stddev_samp(y))
from sample;

select sum( ( x - @ax ) * (y - @ay) ) / ((count(x) -1) * @div) from sample;
+---------------------------------------------------------+
| sum( ( x - @ax ) * (y - @ay) ) / ((count(x) -1) * @div) |
+---------------------------------------------------------+
|                                       0.700885077729073 |
+---------------------------------------------------------+