表格看起来像这样:
Person
first_name
middle_name
last_name
person_ssn
Prisoner
person_ssn
prisoner_id
Visitation
visitor_id
prisoner_id
visitor
visitor_id
first_name
last_name
我有兴趣了解囚犯的姓名以及他们有多少访客。
到目前为止我所得到的:
select person.first_name, person.middle_name, person.last_name, count(visitor_id) AS visitorCount
from prisoner
inner join person on prisoner.person_ssn = person.person_ssn
inner join visitation on prisoner.prisoner_id = visitation.prisoner_id;
这似乎只会导致一名囚犯
最佳答案
您需要一个group by子句。否则，您的查询是返回一行的聚合查询。
select person.first_name, person.middle_name, person.last_name, count(visitor_id) AS visitorCount
from prisoner
inner join person on prisoner.person_ssn = person.person_ssn
inner join visitation on prisoner.prisoner_id = visitation.prisoner_id
group by person.first_name, person.middle_name, person.last_name;
关于mysql - 获取囚犯访客计数，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/23666721/