Calculating percentage within a group

Question

given a table that for the following commands:

select sex, count(*) from my_table group by sex;
select sex, employed, count(*) from my_table group by sex, em         


        

Answer 1

You can do it with a sub-select and a join:

SELECT t1.sex, employed, count(*) AS `count`, count(*) / t2.total AS percent
  FROM my_table AS t1
  JOIN (
    SELECT sex, count(*) AS total 
      FROM my_table
      GROUP BY sex
  ) AS t2
  ON t1.sex = t2.sex
  GROUP BY t1.sex, employed;

I can't think of other approaches off the top of my head.

Answer 2

May be too late, but for upcoming searchers, possible solution could be:

select sex, employed, COUNT(*) / CAST( SUM(count(*)) over (partition by sex) as float)
  from my_table
 group by sex, employed

By IO Statistics this seems to be most effective solution - may be dependant on number of rows to be queried - tested on numbers above ...

The same attitude could be used for getting male / female percentage:

select sex, COUNT(*) / CAST( SUM(count(*)) over () as float)
  from my_table
 group by sex

Regards, Jan