mysql - SQL order by month DESC and year ASC -

i have sql query:

select * `billing`  source = 'voip'  group month(timestamp), year(timestamp)  order month(timestamp) desc, year(timestamp) asc

the above query returning order:

2014-12-01 2014-11-01 2014-10-01 2015-05-01 2015-04-01 2015-03-01 2015-02-01 2015-01-01

i want order so:

2015-05-01 2015-04-01 2015-03-01 2015-02-01 2015-01-01 2014-12-01 2014-11-01 2014-10-01

how ordering date in descending order?

select * `billing`  source = 'voip'  group month(timestamp), year(timestamp)  order min(timestamp) desc;

note: should never use select * group by. should explicitly select columns want, along aggregation functions. like:

select month(timestamp), year(timestamp), count(*) `billing`  source = 'voip'  group month(timestamp), year(timestamp)  order min(timestamp) desc;

(this gets number of records each month.)

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