SQL Query - count - max - sql

Question

I cant manage to come up with a query for a problem. I have three tables
CREATE TABLE institute (
iid INT PRIMARY KEY,
sign VARCHAR(127) UNIQUE,
city VARCHAR(127) NOT NULL,
area INT CHECK (area>0));
CREATE TABLE desease (
did INT PRIMARY KEY,
name VARCHAR(127) UNIQUE,
level INT CHECK (level>0));
CREATE TABLE studies (
did INT,
iid INT,
FOREIGN KEY (did) REFERENCES desease (did),
FOREIGN KEY (iid) REFERENCES institute (iid),
PRIMARY KEY (iid,did));
My question is: What are the names of the deseases by the largest number of institutes from Lisbon (Lisbon beeng the city from institute). This is what i came up with but it doesnt give me the right answer.
SELECT DISTINCT D.name, MAX(I.iid)
FROM desease D, studies S
JOIN institute I ON (S.iid = I.iid)
WHERE I.city = 'Lisboa' AND D.did = S.did
GROUP BY D.nome
HAVING COUNT(I.iid) = MAX(I.city)
As an example : Imagine 5 institutes al with city = 'Lisbon' and with iid A,B,C,D,E respectevely (just for demonstration purposes, I know type is INT); 5 Diseases with name = Z,X,N,V,M respectevely.
Now lets say desease Z,X, and M are studied by institutes A,B,C (in any order), desease N is studied by D(1 inst.) and desease V is studied by E (only one). So the max number of deseases studied by any Lisbon institute is 3 (A,B and C all study 3 deseases) so the table would look like this
Z - 3
X - 3
M - 3
Edit : I managed to found a way to do it. Here is the query that I came up with
SELECT DISTINCT D.name, COUNT(*) AS C
FROM desease D, studies E, institute I
WHERE I.iid = E.iid AND D.did = E.did AND I.city = "Lisboa"
GROUP BY D.name
HAVING C >= ALL (
SELECT COUNT(*)
FROM desease D, studies E, institute I
WHERE I.iid = E.iid AND D.did = E.did AND I.cidade = "Lisboa"
GROUP BY D.name
);

Answer 1

I don't understand structure/problme well enough but I did see that you were mixing joins and had a cross join which would inflate the number of recrds.
SELECT DISTINCT D.name, MAX(I.iid)
FROM desease D
INNER JOIN studies S ON D.iid=S.Did
INNER JOIN institute I ON (S.iid = I.iid)
WHERE I.city = 'Lisboa' AND D.did = S.did
GROUP BY D.nome
HAVING COUNT(I.iid) = MAX(I.city)

Answer 2

This would return a list of disease names that have an institute in Lisbon starting with the one with the greatest number of institutes in Lisbon and going down:
SELECT D.name, COUNT(*) as numberOfInstitutes
FROM desease D
INNER JOIN studies S ON D.did=S.did
INNER JOIN institute I ON (S.iid = I.iid)
WHERE I.city = 'Lisbon'
GROUP BY D.did
ORDER BY COUNT(*) desc
If you need only the one that has the most institutes and you need the rest of the columns from the desease table, you can do this (in Sql Server):
SELECT TOP 1 D.*
FROM desease D
INNER JOIN
(
SELECT D.did, COUNT(*) as numberOfInstitutes
FROM desease D
INNER JOIN studies S ON D.did=S.did
INNER JOIN institute I ON (S.iid = I.iid)
WHERE I.city = 'Lisbon'
GROUP BY D.did
) as tblCount on tblCount.did = D.did
ORDER BY numberOfInstitutes desc

Answer 3

Just a rough guess what you need:
SELECT stu.iid, COUNT(*) AS nstudies
FROM studies stu, institute ins
WHERE stu.iid=ins.iid
AND ins.city='Lisboa'
GROUP BY stu.iid
ORDER BY nstudies DESC;
This should give you a list of institutes that are in Lisboa and the number of studies they did.
SELECT stu.did, COUNT(*) AS ninst
FROM studies stu, institute ins, disease dis
WHERE stu.iid=ins.iid
AND stu.did=dis.did
AND ins.city='Lisboa'
GROUP BY stu.did
ORDER BY ninst DESC;
This gives you a list of deseases and the number of Lisboa instutitues that did it.
Unfortunately your question leaves a lot of room for speculation as to what you need -- maybe you should add some example data and the expected result.