using sub query in sql - mysql

Question

I have 4 Tables:
Travelers (TravelerID,FirstName,LastName)
Guides(GuideID,FirstName,LastName)
Locations(LocationID,LocationName)
Trips(TravelerID,GuideID,LocationID,Stars,StartDate,ReturnDate)
I want to return per guide the name of location which he guided the maximum number of travelers.the result needs to contain all the guides even if they didnt have travelers at all
I have tried to use this sub query but it doesn't work :
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));
the result should be like this:
I will appreciate any help
Edit by me 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))
*/
This data was cropped due to limitations in sqlfiddle length:
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

If you want all the guide but first the guide with max number of teavelers you could use an order by desc eg:
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
If you need the max guide and alos the others name then you could joj the result from the max with all the guides
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