mysql query returns an empty result set - mysql

Question

I have been tasked with a query I am having problems with. Here is the query:
Given a user id and a month, produce a list containing student name, list of files they own (largest to smallest) including total number of files and number of bytes used in a month specified.
Here is what I have so far:
(Select * from htmp_cs368
Join roster_cs368 ON htmp_cs368.userId =
roster_cs368.lastName Where htmp_cs368.userId =
(SELECT lastName FROM roster_cs368 WHERE userId = 'userId' AND htmp_cs368.monthIn = 'monthIn'))
UNION
(Select * from atmp_cs368
JOIN roster_cs368 ON atmp_cs368.userId =
roster_cs368.userId Where roster_cs368.userId =
'userId' AND atmp_cs368.monthIn = 'monthIn') ORDER BY fileSize DESC;
I am getting a result of empty set. My tables are full. I am hoping somone can correct my mistakes.
I have included my schema:
mysql> select * from roster_cs368
-> ;
+--------+-----------+-----------+
| userId | firstName | lastName |
+--------+-----------+-----------+
| apn7cf | Allen | Newton |
| atggg3 | andrew | goebel |
Primary key is userId
mysql> select * from htmp_cs368;
+------------+----------+------------+----------+----------+-------+------+-------+----------------------+
| filePerms | numLinks | userId | idGroup | fileSize | monthIn | day | time | fileName |
+------------+----------+------------+----------+----------+-------+------+-------+----------------------+
| drwx------ | 2 | schulte | faculty | 289 | Nov | 7 | 2011 | Java |
| -rw-r--r-- | 1 | schulte | faculty | 136 | Apr | 29 | 2012 | LD |
| drwxr-xr-x | 3 | schulte | faculty | 177 | Mar | 20 | 2012 | Upgrade |
No primary key here
select * from atmp_cs368;
+------------+----------+--------------+----------+----------+-------+------+-------+-----------------------------+
| filePerms | numLinks | userId | idGroup | fileSize | monthIn | day | time | fileName |
+------------+----------+--------------+----------+----------+-------+------+-------+-----------------------------+
| drwxr-xr-x | 2 | remierm | 203 | 245 | Sep | 17 | 14:40 | 148360_sun_studio_12 |
| drwx---rwx | 31 | antognolij | sasl | 2315 | Oct | 24 | 12:28 | 275 |
| -rwx------ | 1 | kyzvdb | student | 36 | Sep | 19 | 13:05 | 275hh |
No primary key here as either.
I have had very little experience with mysql. I also have to come up with:
If no user id is specified, all files, if no month specified, all users and if neither specified, all months and users.
I am stuck and at a lost. I appreciate any help! Thanks!

Answer 1

You seem to have a number of problems in the SQL.
First
Join roster_cs368 ON htmp_cs368.userId = roster_cs368.lastName
You try to join the userId field to the lastName field, which definitely won't work. It should be userId in both tables.
Then
WHERE userId = 'userId' AND htmp_cs368.monthIn = 'monthIn'
Assuming those really are literal strings, they won't match anything in the table. You need to use a parameterized query, and substitute question marks in the SQL, as in
WHERE userId = ? AND htmp_cs368.monthIn = ?
and provide the actual values to be used in the Java code.
I think you're looking for something along these lines (untested, but this will give you a starting point)
List of files
select r.lastName, r.firstName, t.fileName, t.fileSize
from htmp_cs368 t join roster_cs368 r on t.userId=r.userId
where t.userId=? and t.monthIn=?
order by fileSize desc
Summary:
select r.lastName, r.firstName, count(t.fileName), sum(t.fileSize)
from htmp_cs368 t join roster_cs368 r on t.userId=r.userId
where t.userId=? and t.monthIn=?
group by t.userId
This is a simple approach that does not take into account files appearing and disappearing during a month, but you don't seem to have data in your tables for this.
Also, it's not clear what atmp_cs368 is for, or why the time column in one table seems to have year values.

Answer 2

As pointed out by others you seem to have a number of problems in your SQL. I dont think it can compile as well.
Try:
SELECT r.userId, files.*
FROM roster_cs368 AS r
JOIN (
Select * from htmp_cs368 WHERE userId = 'userId' AND monthIn = 'monthIn'
UNION
Select * from atmp_cs368 Where userId = 'userId' AND monthIn = 'monthIn'
) AS files ON files.userId = r.userId
ORDER BY files.fileSize DESC;
You need only one JOIN. This lists users and all their files. And take care to equate apples to apples (userId != lastName).
Now to get count of files and file sizes etc you need a GroupBy effectively. But you cannot list files and get count of files together "easily". It will have to be one way or other. Just for the count you can use Jim's solution.

Answer 3

This JOIN looks a tad suspicious...
JOIN roster_cs368 ON htmp_cs368.userId = roster_cs368.lastName
Even if userId in htmp_cs368 has an equivalent value in the lastName column of roster_cs368, this is very bad form. JOINS should typically be done on like-named columns.
If these two columns are unrelated (it's hard to tell when roster_cs368 also has a userId column), then that would be at least part of your problem.
Also, htmp_cs368.monthIn = 'monthIn' doesn't make sense. This won't match anything in that column either.