如何根据其他表中的值的数量从表中获取记录

Question

所以我有表A（城市）

id，city，country

表B（旅行）

id，trip，city，active

我想选择所有使用MySQL的城市，但是当活动为1时，他们会在表B中显示它们的次数。

提前致谢

Answer 1

您可以尝试查询和比较效果

select a.city, count(*) as cnt
from cities as a
left join
trips as b
on (a.id = b.city and b.active=1)
group by a.city
order by 2 desc

或

select
    a.city,
    (select count(*) from trips as b where a.id = b.city and b.active=1) as cnt
from cities as a
order by 2 desc

Answer 2

然后在B上离开加入A并在join子句中包含active = 1条件，并按城市分组。

select a.city, count(b.city) as cnt
from a
left join b on a.id=b.city and b.active=1
group by a.id, a.city
order by count(b.city) desc