How to write a Ecto query that does a group_by MONTH on a datetime field

问题

I am doing a ecto query and am trying to group by q.created_date. This query successfully does the GROUP BY but it does it by the second. I am trying to group by month instead.

MYQUERY |> group_by([q], [q.created_date, q.id])

Is there something like:

MYQUERY |> group_by([q], [month(q.created_date), q.id])

回答1:

You can use fragment with date_part('month', field) to extract the month in PostgreSQL: fragment("date_part('month', ?)", p.inserted_at))

iex(1)> from(Post) |> select([p], p.inserted_at) |> Repo.all
[debug] QUERY OK db=1.1ms queue=0.1ms
SELECT p0."inserted_at" FROM "posts" AS p0 []
[#Ecto.DateTime<2000-01-01 00:00:00>, #Ecto.DateTime<2000-02-02 00:00:00>,
 #Ecto.DateTime<2000-03-03 00:00:00>, #Ecto.DateTime<2000-04-04 00:00:00>,
 #Ecto.DateTime<2000-04-02 00:00:00>, #Ecto.DateTime<2000-03-02 00:00:00>,
 #Ecto.DateTime<2000-02-02 00:00:00>, #Ecto.DateTime<2000-01-02 00:00:00>]
iex(2)> from(Post) |> group_by([p], fragment("date_part('month', ?)", p.inserted_at)) |> select([p], count("*")) |> Repo.all
[debug] QUERY OK db=1.7ms
SELECT count('*') FROM "posts" AS p0 GROUP BY date_part('month', p0."inserted_at") []
[2, 2, 2, 2]

来源：https://stackoverflow.com/questions/38648371/how-to-write-a-ecto-query-that-does-a-group-by-month-on-a-datetime-field