Generated columns using aggregate functions

Question

The starting point

Suppose I have a table devTest that looks like this:

+----+------+  
| id | j    |  
+----+------+  
|  1 |    5 |  
|          


        

Answer 1

This error makes sense because any change in your table (say you add a j with value 0) would update your average, and this in turn would change all your generated columns. So this would be quite a bit of work for the query engine.

Another solution would be to define a view instead:

CREATE VIEW j_dev (id, j, j_dev) AS 
SELECT id, j, 
       (j - avg(j)) / std(j) as j_dev
FROM devTest

(not sure about the create view syntax, correct me if I'm wrong)

Answer 2

In standard SQL you'd do:

select id, j, (j - avg(j) over ()) / std(j) over () as jdev
from devtest;

But MySQL doesn't support analytic window functions such as AVG OVER. So in MySQL, you must select the aggregation values separately:

select d.id, d.j, (d.j - agg.javg) / agg.jstd as jdev
from devtest d
cross join (select avg(j) as javg, std(j) as jstd from devtest) agg;

Then create a view as Benjamin Crouzier suggests in his answer:

CREATE VIEW v_devtest AS
  select d.id, d.j, (d.j - agg.javg) / agg.jstd as jdev
  from devtest d
  cross join (select avg(j) as javg, std(j) as jstd from devtest) agg;

A computed column can only calculate its value from other values in the same record. So what you are trying to do cannot be done with a calculated column.