SQL Query causing me lack of sleep - sql

Question

So I am revising for an exam and have struck a big rock in the SQL river (or waste ground)
I made the following tables and inserted the following data:
create table Permissions
(
fileName VARCHAR(40),
userID VARCHAR (16),
type VARCHAR(10),
startdate DATE,
duration NUMBER (5),
constraint Pri_key PRIMARY KEY (userID,fileName)
);
create table Files
(
name VARCHAR(20),
fsize INT,
numberofpermissions INT,
constraints PRI_KEY2 PRIMARY KEY (name)
);
create table Users
(
id VARCHAR(20),
password VARCHAR (20),
constraint Pri_key3 PRIMARY KEY (id)
);
-- after all tables create:
alter table Permissions
add constraint Forn_key FOREIGN KEY (userID) REFERENCES Users(id)
INITIALLY DEFERRED DEFERRABLE;
alter table Permissions
add constraint Forn_key2 FOREIGN KEY (filename) REFERENCES Files(name)
INITIALLY DEFERRED DEFERRABLE;
insert into Permissions VALUES ('Agenda','Jones','read','19-JAN-10',30);
insert into Permissions VALUES ('Agenda','Chun','read','19-JAN-10',30);
insert into Permissions VALUES ('Agenda','Rashid','write','17-JAN-10',50);
insert into Permissions VALUES ('Finance','Chun','write','05-DEC-09',50);
insert into Permissions VALUES ('AnnualReport','Jones','write','12-DEC-09',50);
insert into Users VALUES ('Jones', 'duck');
insert into Users VALUES ('Chun', 'tiger');
insert into Users VALUES ('Adams', 'shark');
insert into Users VALUES ('Rashid', 'puma');
insert into Files VALUES ('Agenda', 32, 3);
insert into Files VALUES ('FinanceTables',645, 0);
insert into Files VALUES ('Finance', 120, 1);
insert into Files VALUES ('AnnualReport', 1205, 1);
commit;
I Am now trying to write a SQL command to display for each user who has
permissions for files of a total size of more than 50: the user’s
id, the total size of all the files the user has permissions for, and
the user’s password.
Here is what I have so far but when i try to add anything in to get the password, SQL+ throws up a hissy fit and there will be a hole in my screen soon!
SELECT permissions.userID, sum(fsize) AS Totalsize
FROM files, permissions
where permissions.filename = files.name
group by permissions.userid
having SUM(fsize) > 50;

Answer 1

In oracle, you need to specify the entire group by
SELECT permissions.userID, users.password, sum(fsize) AS Totalsize
FROM files, permissions, users
where permissions.filename = files.name
and users.id = permissions.userID
group by permissions.userid, permissions.password
having SUM(fsize) > 50;
This is different from MySQL where the group by can be implied, but this is more correct.

Answer 2

Join against the user table, and add the password to the group by clause:
SELECT permissions.userID, users.password, sum(fsize) AS Totalsize
FROM files, permissions, users
where permissions.filename = files.name
and users.id = permissions.userID
group by permissions.userid, users.password
having SUM(fsize) > 50;

Answer 3

Use the JOIN syntax. It complies with ANSI SQL. Joining using WHERE is an old syntax that should not be used any more.
SELECT
u.id AS userid, u.password, SUM(f.fsize) AS Totalsize
FROM
users u
INNER JOIN permissions p
ON u.id = p.userID
INNER JOIN files f
ON p.filename = f.name
GROUP BY
u.id, u.password
HAVING
SUM(f.fsize) > 50;
Note that conditions based on aggregation functions must be placed in the HAVING clause. The difference between the WHERE and the HAVING clause is, that WHERE is executed before grouping and HAVING is executed after grouping.
Also, the GROUP BY clause must contain all the expressions from the SELECT-list that don't have an aggregation function.

Answer 4

This should work:
SELECT Users.id, Users.password, Totalsize
FROM Users
INNER JOIN
(SELECT userId, SUM(fSize) AS TotalSize
FROM permissions
INNER JOIN files ON permissions.filename = files.name
group by permissions.userid
having SUM(fsize) > 50) t ON t.userId = Users.Id
This could be done without the inner join of course, however if you gonna need more fields this makes it easier to add them without having to add the extra fields to the group by clause.