Mysql : left outer join issue -

i have 2 tables :
income(month, amount)
expenditure(month, amount)

i need display each month of table income amount minus expenditure amount (if exists).
think need use left outer join between 2 tables, like

select income.amount - expenditure.amount  income left outer join expenditure on income.month = expenditure.month 

but don't know how http://sqlfiddle.com/#!9/7a7d5/1

if can on sqlfiddle
thanks.

i assume if there no income should treated 0, likewise expenditure.

mysql doesn't have full outer join can similar:

select month, sum(amount)     (select month, income amount     income     union     select month, - expenditure amount     expenditure) group month; 

this creates union of 2 tables (with expenditure negative simplicity). sums amounts , groups month.

example: http://sqlfiddle.com/#!9/7a7d5/14