在sql中使用子查询

Question

我有4个表：

Travelers (TravelerID,FirstName,LastName) 
Guides(GuideID,FirstName,LastName) 
Locations(LocationID,LocationName) 
Trips(TravelerID,GuideID,LocationID,Stars,StartDate,ReturnDate)

我想按指南返回他引导最多旅行者的位置名称。结果需要包含所有指南，即使他们根本没有旅行者

我试图使用此子查询，但它不起作用：

SELECT G.FirstName,L.LocationName,count(distinct(TravelerID))as 
number_of_travelers_per_guide 
FROM Guides AS G 
LEFT JOIN Trips AS T USING (GuideID) 
LEFT JOIN Locations AS L USING (LocationID)
GROUP BY G.FirstName,L.LocationName 
HAVING max((SELECT T1.number_of_travelers_per_guide 
        FROM Trips AS T1 
        WHERE T.GuideID=T1.GuideID));

结果应该是这样的：

我将不胜感激任何帮助

---

由我编辑Patrick Artner：

/* data creation script - python 3.6.2

import random

random.seed(815) # fixed 0815 seed

Travelers = [[x,'a' + str(x), 'A' + str(x)] for x in range(20)] 

Guides = [ [x,"guide_" + str(x), "G_" + str(x)] for x in range(10)]

Locations = [[x, "location_" + str(x)] for x in range(6)] 

# Trips(TravelerID,GuideID,LocationID,Stars,StartDate,ReturnDate)
Trips = []
for n in range(300):
    Trips.append([random.choice(range(20)),random.choice(range(10)),random.choice(range(6)),random.randint(1,6),None, None])


def prnList(lst):
    def prn(lst):
        for i in lst:
            yield " ,".join([str(x) for x in i]) + "\n"
        yield "\n\n" 
    return "".join(prn(lst))

with open("demodata.txt","w") as f:
    f.write("Travelers\nTravelerID,FirstName,LastName\n")
    f.write(prnList(Travelers))
    f.write("Guides\nGuideID,FirstName,LastName\n") 
    f.write(prnList(Guides))
    f.write("Locations\nLocationID,LocationName\n") 
    f.write(prnList(Locations)) 
    f.write("Trips\nTravelerID,GuideID,LocationID,Stars,StartDate,ReturnDate\n")
    f.write(prnList(Trips))
*/

由于sqlfiddle长度的限制，此数据被裁剪：

CREATE TABLE   Travelers     (`TravelerID` int, `FirstName` varchar(3), `LastName` varchar(3));

INSERT INTO   Travelers    (`TravelerID`, `FirstName`, `LastName`)
VALUES
    (0, 'a0', 'A0'),
    (1, 'a1', 'A1'),
    (2, 'a2', 'A2'),
    (3, 'a3', 'A3'),
    (4, 'a4', 'A4'),
    (5, 'a5', 'A5'),
    (6, 'a6', 'A6'),
    (7, 'a7', 'A7'),
    (8, 'a8', 'A8'),
    (9, 'a9', 'A9'),
    (10, 'a10', 'A10'),
    (11, 'a11', 'A11'),
    (12, 'a12', 'A12'),
    (13, 'a13', 'A13'),
    (14, 'a14', 'A14'),
    (15, 'a15', 'A15'),
    (16, 'a16', 'A16'),
    (17, 'a17', 'A17'),
    (18, 'a18', 'A18'),
    (19, 'a19', 'A19')
;


CREATE TABLE Guides     (`GuideID` int, `FirstName` varchar(7), `LastName` varchar(3));

INSERT INTO Guides     (`GuideID`, `FirstName`, `LastName`)
VALUES
    (0, 'guide_0', 'G_0'),
    (1, 'guide_1', 'G_1'),
    (2, 'guide_2', 'G_2'),
    (3, 'guide_3', 'G_3'),
    (4, 'guide_4', 'G_4'),
    (5, 'guide_5', 'G_5'),
    (6, 'guide_6', 'G_6'),
    (7, 'guide_7', 'G_7'),
    (8, 'guide_8', 'G_8'),
    (9, 'guide_9', 'G_9')
;


CREATE TABLE Locations    (`LocationID` int, `LocationName` varchar(10));

INSERT INTO Locations    (`LocationID`, `LocationName`)
VALUES
    (0, 'location_0'),
    (1, 'location_1'),
    (2, 'location_2'),
    (3, 'location_3'),
    (4, 'location_4'),
    (5, 'location_5')
;


CREATE TABLE Trips    (`TravelerID` int, `GuideID` int, `LocationID` int, `Stars` int, `StartDate` varchar(4), `ReturnDate` varchar(4));

INSERT INTO Trips    (`TravelerID`, `GuideID`, `LocationID`, `Stars`, `StartDate`, `ReturnDate`)
VALUES
    (0, 4, 0, 5, 'None', 'None'),
    (9, 5, 3, 1, 'None', 'None'),
    (16, 9, 0, 6, 'None', 'None'),
    (6, 3, 2, 4, 'None', 'None'),
    (5, 0, 2, 4, 'None', 'None'),
    (1, 4, 4, 5, 'None', 'None'),
    (5, 4, 1, 1, 'None', 'None'),
    (8, 7, 3, 3, 'None', 'None'),
    (8, 2, 1, 5, 'None', 'None'),
    (5, 6, 3, 5, 'None', 'None'),
    (2, 8, 1, 3, 'None', 'None'),
    (17, 0, 1, 4, 'None', 'None'),
    (6, 8, 3, 4, 'None', 'None'),
    (0, 3, 3, 2, 'None', 'None'),
    (4, 8, 3, 4, 'None', 'None'),
    (16, 5, 0, 6, 'None', 'None'),
    (10, 9, 5, 4, 'None', 'None'),
    (19, 8, 0, 4, 'None', 'None'),
    (7, 7, 4, 3, 'None', 'None'),
    (7, 4, 1, 5, 'None', 'None'),
    (13, 7, 5, 6, 'None', 'None'),
    (0, 9, 5, 2, 'None', 'None'),
    (4, 5, 5, 5, 'None', 'None'),
    (19, 1, 2, 3, 'None', 'None'),
    (2, 6, 1, 1, 'None', 'None'),
    (13, 3, 0, 4, 'None', 'None'),
    (8, 0, 0, 2, 'None', 'None'),
    (18, 6, 2, 5, 'None', 'None'),
    (14, 4, 5, 3, 'None', 'None'),
    (12, 8, 1, 6, 'None', 'None'),
    (8, 1, 3, 4, 'None', 'None'),
    (15, 1, 5, 2, 'None', 'None'),
    (5, 1, 4, 6, 'None', 'None'),
    (5, 5, 1, 1, 'None', 'None'),
    (14, 6, 0, 5, 'None', 'None'),
    (3, 8, 3, 6, 'None', 'None'),
    (3, 1, 1, 4, 'None', 'None'),
    (17, 1, 0, 4, 'None', 'None'),
    (6, 4, 0, 1, 'None', 'None'),
    (16, 6, 5, 6, 'None', 'None'),
    (6, 8, 3, 1, 'None', 'None'),
    (13, 8, 1, 1, 'None', 'None'),
    (13, 3, 5, 5, 'None', 'None'),
    (12, 4, 4, 2, 'None', 'None'),
    (13, 8, 3, 3, 'None', 'None'),
    (3, 1, 1, 1, 'None', 'None'),
    (6, 4, 2, 2, 'None', 'None'),
    (0, 8, 0, 6, 'None', 'None'),
    (3, 8, 3, 2, 'None', 'None'),
    (17, 7, 1, 3, 'None', 'None'),
    (0, 4, 4, 1, 'None', 'None'),
    (13, 0, 2, 6, 'None', 'None'),
    (5, 5, 2, 6, 'None', 'None'),
    (14, 8, 0, 2, 'None', 'None'),
    (12, 0, 1, 2, 'None', 'None'),
    (3, 7, 3, 1, 'None', 'None'),
    (8, 2, 0, 2, 'None', 'None'),
    (17, 0, 4, 4, 'None', 'None'),
    (4, 6, 0, 4, 'None', 'None'),
    (2, 2, 1, 1, 'None', 'None'),
    (9, 6, 4, 6, 'None', 'None'),
    (11, 1, 4, 3, 'None', 'None'),
    (6, 5, 2, 3, 'None', 'None'),
    (1, 5, 5, 5, 'None', 'None'),
    (13, 6, 3, 3, 'None', 'None'),
    (15, 4, 5, 2, 'None', 'None'),
    (5, 3, 5, 2, 'None', 'None'),
    (2, 5, 5, 5, 'None', 'None'),
    (6, 2, 1, 1, 'None', 'None'),
    (19, 0, 1, 4, 'None', 'None'), 
    (15, 3, 3, 1, 'None', 'None'),
    (13, 1, 5, 1, 'None', 'None'),
    (14, 8, 3, 4, 'None', 'None'),
    (19, 5, 4, 5, 'None', 'None'),
    (17, 1, 0, 5, 'None', 'None'),
    (6, 8, 3, 4, 'None', 'None'),
    (0, 4, 4, 2, 'None', 'None'),
    (16, 5, 3, 4, 'None', 'None'),
    (13, 4, 5, 6, 'None', 'None'),
    (0, 8, 4, 3, 'None', 'None'),
    (10, 0, 3, 3, 'None', 'None'),
    (0, 8, 0, 4, 'None', 'None'),
    (9, 6, 5, 1, 'None', 'None'),
    (9, 7, 3, 5, 'None', 'None'),
    (18, 2, 4, 3, 'None', 'None'),
    (15, 9, 3, 5, 'None', 'None'),
    (5, 5, 3, 3, 'None', 'None'),
    (17, 4, 1, 4, 'None', 'None'),
    (7, 6, 4, 5, 'None', 'None'),
    (7, 9, 3, 5, 'None', 'None'),
    (12, 3, 3, 3, 'None', 'None'),
    (13, 2, 3, 6, 'None', 'None'),
    (17, 8, 5, 5, 'None', 'None'),
    (15, 8, 0, 3, 'None', 'None'),
    (3, 3, 2, 4, 'None', 'None'),
    (11, 1, 2, 5, 'None', 'None'),
    (17, 1, 0, 3, 'None', 'None'),
    (17, 4, 0, 5, 'None', 'None'),
    (1, 5, 1, 4, 'None', 'None'),
    (16, 8, 4, 4, 'None', 'None'),
    (15, 5, 3, 3, 'None', 'None'),
    (17, 9, 4, 1, 'None', 'None'),
    (1, 8, 2, 5, 'None', 'None'),
    (15, 9, 2, 6, 'None', 'None'),
    (17, 2, 1, 1, 'None', 'None'),
    (12, 1, 2, 6, 'None', 'None'),
    (6, 9, 1, 3, 'None', 'None'),
    (5, 1, 3, 5, 'None', 'None'),
    (8, 1, 5, 2, 'None', 'None'),
    (12, 9, 1, 5, 'None', 'None'),
    (3, 4, 0, 5, 'None', 'None'),
    (11, 6, 2, 4, 'None', 'None'),
    (8, 0, 5, 1, 'None', 'None'),
    (5, 1, 1, 1, 'None', 'None'),
    (15, 3, 0, 3, 'None', 'None'),
    (15, 1, 2, 4, 'None', 'None'),
    (2, 6, 1, 5, 'None', 'None'),
    (19, 7, 4, 6, 'None', 'None'),
    (2, 2, 4, 1, 'None', 'None'),
    (19, 2, 2, 6, 'None', 'None'),
    (10, 4, 4, 2, 'None', 'None'),
    (0, 1, 1, 1, 'None', 'None'),
    (7, 2, 4, 3, 'None', 'None'),
    (16, 5, 3, 4, 'None', 'None'),
    (11, 3, 4, 3, 'None', 'None'),
    (15, 1, 2, 5, 'None', 'None'),
    (9, 4, 0, 3, 'None', 'None'),
    (16, 3, 5, 5, 'None', 'None'),
    (7, 8, 4, 6, 'None', 'None'),
    (14, 5, 0, 5, 'None', 'None'),
    (19, 6, 3, 1, 'None', 'None'),
    (17, 5, 3, 5, 'None', 'None'),
    (12, 7, 0, 5, 'None', 'None'),
    (7, 0, 1, 2, 'None', 'None'),
    (0, 1, 4, 4, 'None', 'None'),
    (16, 2, 0, 3, 'None', 'None')

;

Answer 1

如果你想要所有的指南，但首先是最大数量的旅行者指南你可以使用desc的订单，例如：

  SELECT G.FirstName, L.LocationName, count(distinct(TravelerID))as number_of_travelers_per_guide 
  FROM Guides AS G 
  LEFT JOIN Trips AS T USING (GuideID) 
  LEFT JOIN Locations AS L USING (LocationID)
  GROUP BY G.FirstName,L.LocationName 
  ORDER BY number_of_travelers_per_guide DESC

如果您需要最大指南和其他名称，那么您可以使用所有指南来获取最大结果

select Guides.FirstName, my_t2.LocationName,  my_t2.number_of_travelers_per_guide 
from (

    SELECT G.GuideId, G.FirstName,L.LocationName, count(distinct T.TravelerID ) as number_of_travelers_per_guide 
    FROM Guides AS G 
    LEFT JOIN Trips AS T USING (GuideID) 
    LEFT JOIN Locations AS L USING (LocationID)
    GROUP BY G.FirstName,L.LocationName 
    HAVING count(distinct TravelerID ) = ( 
                select max(my_count) from (SELECT count(distinct T.TravelerID) my_count
                FROM Guides AS G 
                LEFT JOIN Trips AS T USING (GuideID) 
                LEFT JOIN Locations AS L USING (LocationID)
                GROUP BY G.FirstName, L.LocationName ) my_t )
  )  my_t2
left join Guides on Guides.GuideID = my_t2.GuideID