sql - count(*) 在三个表上

Question

我有三张桌子

State(stateid, statename)
District(dict_id,dict_name,stateid)
city(city_id,dict_id,city_name) 

我需要计算该州的州名和城市。是否可以在单个查询中获取 Statename、district_count、city_count？

Answer 1

这应该这样做：

SELECT  S.Statename, 
        COUNT(DISTINCT C.dict_id) district_count,
        COUNT(DISTINCT C.city_id) city_count
FROM City C
INNER JOIN District D
    ON C.dict_id = D.dict_id
INNER JOIN State S
    ON S.stateid = D.stateid
GROUP BY S.Statename

Answer 2

你需要使用GROUP BY. 尝试这样的事情：

SELECT S.StateId, S.StateName, COUNT(DISTINCT D.Dict_Id) as DictCount, COUNT(DISTINCT C.City_Id) as CityCount
FROM State S
  LEFT JOIN District D ON S.StateId = D.StateId
  LEFT JOIN City C ON D.Dict_Id = C.Dict_Id
GROUP BT  S.StateId, S.StateName

祝你好运。