我有如下所示的 SQL 数据
events
id name capacity
1 Cooking 10
2 Swimming 20
3 Archery 15
registrants
id name
1 Jimmy
2 Billy
3 Sally
registrant_event
registrant_id event_id
1 3
2 3
3 2
我想选择“事件”中的所有字段以及一个附加字段，即当前注册该事件的人数。在这种情况下，射箭有 2 个注册人，游泳有 1 个，烹饪有 0 个。
我想这可以在单个查询中完成，但我不确定语法是否正确。 如何编写查询来获取该数据？
更新：感谢您的精彩回答，你们都摇滚！
【问题讨论】：
标签： sql database count subquery
SELECT e.*, ISNULL(ec.TotalRegistrants, 0) FROM events e LEFT OUTER JOIN
(
SELECT event_id, Count(registrant_id) AS TotalRegistrants
FROM registrant_event
GROUP BY event_id
) ec ON e.id = ec.event_id
【讨论】：
SELECT Events.ID, Events.Name, Events.Capacity,
ISNULL(COUNT(Registrant_Event.Registrant_ID), 0)
FROM Events
LEFT OUTER JOIN Registrant_Event ON Events.ID = Registrant_Event.Event_ID
GROUP BY Events.ID, Events.Name, Events.Capacity
【讨论】：
select d.id1, d.name, d.cappacity, count(d.b_id) as number_of_people
from (select eve.id1,eve.name,eve.cappacity,re_eve.b_id
from eve left join re_eve on eve.id1 = re_eve.b_id) d
group by d.id1, d.name, d.cappacity
【讨论】：
我在 Oracle 11g 中对此进行了测试，它似乎可以工作
SELECT e.id, e.name, e.capacity, COUNT(re.event_id)
FROM events e
LEFT JOIN registrant_event re
ON e.id = re.event_id
GROUP BY e.id, e.name, e.capacity
【讨论】：
select e.id, e.name, e.capacity, IsNull(re.eventCount,0) from events e
left join (
select count(event_id) as eventCount, event_id from registrant_event group by event_id
) re
on e.id = re.event_id【讨论】：
SELECT e.id, count(*) AS NumRegistrants
FROM events e
LEFT JOIN registrant_event re ON re.event_id=e.id
GROUP BY e.id
请注意，对于没有注册人的事件，这将返回 1 而不是 0。要让它显示 0，您必须变得更复杂一些：
SELECT *,
(SELECT COUNT(*) FROM registrant_event WHERE event_id=id) AS NumRegistrants
FROM events
【讨论】：
SELECT
events.*
, COUNT(registrant_event.registrant_id) AS registrantsCount
FROM events
LEFT JOIN registrant_event ON events.id = registrant_event.event_id
GROUP BY events.id
【讨论】：