即使在0 postgres时也选择计数

Question

我想在日期之后计算一堆条目。但是，我也希望得到结果为0.但是当我按日期子句添加过滤器时，当前查询会丢弃所有0结果。任何想法如何获得这0个条目？

select distinct (e.customer_id), count(m.id) as msgs from eng_msgs e
join messages m on e.customer_id = m.customer_id
where e.customer_id = m.customer_id
and m.created_at > e.created_at -- this line removes 0 results
group by e.customer_id 

Answer 1

您需要outer join条件：

select e.customer_id, count(m.id) as msgs
from
    eng_msgs e
    left outer join
    messages m on
        e.customer_id = m.customer_id
        and
        m.created_at > e.created_at
group by e.customer_id