Postgresql 使用generate_series从两个表中选择count

我有两张桌子，问答。我想从这些表中选择两个count（*），每个月生成一个系列

SELECT to_char(s.tag,'yyyy-mm') AS month
     , count(t.id) AS questions_count
FROM  (
   SELECT generate_series(min(created_time)::date
                        , current_date::date
                        , interval '1 month'
          )::date AS tag
   FROM   core.questions t
    where t.created_by=2
   ) s
LEFT   JOIN core.questions t ON t.created_time::date = s.tag

GROUP  BY 1
ORDER  BY 1;

我想从这样的两个表中得到一个结果

month      questions_count   answer_count 
"2006-12" | 4              | 1
"2007-01" | 1              | 2
"2007-02" | 1              | 3

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透视/交叉选项卡/条件聚合