What´s wrong in my query? - sql

Question

I have a query that I can get right. So here is my scenario:
I have 1 parent table called 'Couriers', then 1 courier may have 1 or many 'Messengers' then Couriers has a 2nd table called 'Control' where I control how many messengers can a courier have.
The structures goes like this:
'Courier'
idCourier(pk)
181
162
101
'Messenger'
idMessenger (pk), idCourier (fk)
1 101
2 181
3 101
4 101
5 162
'Control'
idCourier(pk, fk), maxNumberOfMessengers, idSupervisor
181 20 146
181 20 149
162 10 129
162 10 130
162 10 138
162 10 139
101 0 83
101 0 86
So with the next query:
SELECT COUNT(G.idCourier) AS NUM_COURIER, G.idMessenger, SUM(T.maxNumberOfMessengers) AS TOTAL
FROM Courier M
LEFT JOIN Messenger G ON M.idCourier = G.idCourier
LEFT JOIN Control T ON T.idCourier = G.idCourier AND T.idCourier = M.idCourier
GROUP BY G.idMessenger
I get:
NUM_COURIER G.idMessenger TOTAL
2 181 40
4 162 40
6 101 0
And I was expecting:
NUM_COURIER G.idMessenger TOTAL
1 181 40
3 162 40
1 101 0
What am I missing or doing wrong in my query???
Thanks in advance!!:)

Answer 1

How many messengers do we have per courier?
SELECT idCourier, COUNT(*) CourierCount FROM Messenger GROUP BY idCourier
Now join this and compare to control:
SELECT CO.idcourier,
CO.maxnumberofmessengers,
CC.couriercount
FROM control CO
INNER JOIN (SELECT idcourier,
Count(*) CourierCount
FROM messenger
GROUP BY idcourier) CC
ON CC.idcourier = CO.idcourier

Answer 2

I am not familiar with Oracle so I hope this is helpful as this is how you would do this with MS Sql.
SELECT COUNT(G.idCourier) AS NUM_COURIER,
m.idcourier,
(
SELECT sum(maxnumberofmessengers)
FROM CONTROL con
WHERE con.idcourier = m.idcourier
GROUP BY con.idcourier
) AS TOTAL
FROM Courier M
JOIN Messenger G ON M.idCourier = G.idCourier
GROUP BY m.idcourier

Answer 3

My previous query incorrectly assumed that we needed to count the number of courier entries in the Control table, while also summing the number of messengers.
SELECT COUNT(T.idCourier) AS NUM_COURIER, T.idCourier, SUM(T.maxNumberOfMessengers) AS TOTAL
FROM Control T
GROUP BY T.idCourier
I see now that we need to get the number of Messengers. So a sub query is required:
SELECT Couriers.CourierCount as NUM_COURIER, Couriers.idCourier, MaxMessengers.TOTAL
FROM
(SELECT M.idCourier, COUNT(*) CourierCount FROM Messenger M GROUP BY M.idCourier) Couriers
JOIN(SELECT C.idCourier, SUM(C.maxNumberOfMessengers) AS TOTAL From Control C GROUP BY C.idCourier) MaxMessengers
ON MaxMessengers.idCourier = Couriers.idCourier

Answer 4

courier table and control table is connected by idcourier and courier table and messenger table is also connected by idcourier but messenger table and control table are not connected at all. Another thing is idcourier alone can not be pk in control table as per its data. it has duplicate data.
select t1.idcourier,cnt,sm
from (
SELECT COURIER.IDCOURIER,
COUNT(MESSENGER.IDMESSENGER) as cnt
FROM COURIER
left JOIN MESSENGER
ON COURIER.IDCOURIER = MESSENGER.IDCOURIER
GROUP BY COURIER.IDCOURIER ) t1
inner join (
SELECT COURIER.IDCOURIER,
SUM(CONTRAL.MAXNUMBEROFMESSENGERS) as sm
FROM COURIER
left JOIN CONTRAL
ON COURIER.IDCOURIER = CONTRAL.IDCOURIER
GROUP BY COURIER.IDCOURIER )t2 on t1.idcourier = t2.idcourier;