如何使用sqlalchemy进行平均计数？

Question

如何计算SQLAlchemy的平均值？我尝试了很多东西，但是都失败了。代码：

month = sqlalchemy.func.date_trunc('month', Complaint.date)
records_count = sqlalchemy.sql.func.count(ClientGroupRecord.id)
complained_clients = session.query(Friend, records_count.label('count'), month). \
    filter(Friend.friend_id == friend_id). \
    join(Complaint, Complaint.friend == Friend.friend_id). \
    join(ClientGroup, Complaint.client_group == ClientGroup.client_group_id). \
    join(ClientGroupRecord, ClientGroup.client_group_id == ClientGroupRecord.client_group_id). \
    join(Client, Client.client_id == ClientGroupRecord.client_id). \
    group_by(month, Friend.friend_id)

records_avg = sqlalchemy.sql.func.avg(records_count)

result = ???

在result中，我不需要Friend，但要在该列上标记为avg并由count列分组的month。

Answer 1

[一种选择是使用窗口函数，如果您的DBMS支持它们的话–基于date_trunc的使用，我认为您正在使用PostgreSQL。之所以可行，是因为在GROUP BY产生了组行之后才对窗口函数进行求值。因此将计数包装在AVG(...) OVER ()中，其中窗口是整个结果集：

from sqlalchemy import func

month = func.date_trunc('month', Complaint.date)
records_count = func.count(ClientGroupRecord.id)
# Create a window of the whole set of results
records_avg = func.avg(records_count).over()

complained_clients = session.query(records_avg.label('avg'),
                                   month.label('month')). \
    select_from(Friend). \
    join(Complaint, Complaint.friend == Friend.friend_id). \
    join(ClientGroup, Complaint.client_group == ClientGroup.client_group_id). \
    join(ClientGroupRecord, ClientGroup.client_group_id == ClientGroupRecord.client_group_id). \
    join(Client, Client.client_id == ClientGroupRecord.client_id). \
    filter(Friend.friend_id == friend_id). \
    group_by(month). \
    all()