Nested SELECT to find highest value and display 0 if not found - mysql

Question

Thank you stackoverflow community! I have learned SO much from you over the years and I've recently created an account. I hope the answer to this question isn't obviously somewhere already, but I am going to go crossed eyed if I read another post. Here's my problem:
I recently used a nested SELECT to get the highest score for each of my students from a table. I did so by a little trick another post taught me. I can't find the exact post I learned it from, but here is a snippet that's essentially the same trick. I imagine, if you are well versed in sql, it's nothing new to you:
SELECT id, authorId, answer, votes
FROM ( SELECT id, authorId, answer, votes
FROM answers
ORDER BY votes DESC) AS h
GROUP BY authorId
The ORDER BY ____ DESC makes the last value, the highest overwrite all previous, so you end up with only it...if I understand correctly. So, that was great and I tailored it to my needs. The only problem is, now, I'd like to add one more feature to it and I'm racking my brain cells over it. I'm hoping some generous person will just straighten me out. I want to get a complete list of students from my "rosters" table and if there is no score for a given student, in my "holder" table, I'd like it to display a "0". Here is what I have, and I don't know exactly how to tweak it to do just that:
SELECT *
FROM (
SELECT
holder.id,
#IFNULL(holder.score, 0) AS score,
holder.score AS score,
holder.total,
holder.student_id AS stu_id,
holder.date AS date,
users.firstname AS first,
users.lastname AS last,
users.stu_number AS stuno,
assignments.name AS test,
rosters.p_id,
preps.period AS period,
preps.user
FROM holder
JOIN rosters
ON rosters.stu_id = holder.student_id
JOIN users
ON users.id = holder.student_id
JOIN assignments
ON assignments.id = holder.t_id
JOIN preps
ON preps.id = rosters.p_id
WHERE holder.visible = 0
AND preps.user = 1
AND assignments.user = 1
AND holder.t_id = 1
AND preps.period = 2
ORDER BY score DESC
) x
GROUP BY stuno
ORDER BY last
You can see that line I commented out is one of my feeble attempts to get it to display a "0" if NULL, but it's not working. I get a complete list, but if the score isn't found for a student, that student isn't showing up in my list. Anyone have a solution/idea for me to try?
Am I overusing JOINs and making my life harder than it needs to be? I'm mostly self-taught, so I know I have some holes in the fundamentals. It hasn't stopped me from creating some crazy cool projects though...but every now and then I'm sure I'm causing myself some unnecessary grief.
///////////////////////////////////////////////
Here is what I have done with the answer below, so that it grabs info from my tables:
SELECT au.stu_id,
COALESCE(t.id, 0) as id,
COALESCE(t.score , 0) as score
FROM rosters au
LEFT JOIN (
SELECT *
FROM (
SELECT a.*,
#rownum := if(#prev_value = student_id,
#rownum + 1,
1) rn,
#prev_value := student_id as prev
FROM holder a,
(SELECT #rownum := 0, #prev_value := '') r
ORDER BY student_id, score DESC
) T
WHERE T.rn = 1) T
ON au.stu_id = T.student_id
So, this is working great, except it doesn't show students who don't have scores for a given test. If their score isn't found in the "holder" table, I'd like it to show up as a "0".
/////////////////
Wait a minute! I may have mispoke...I think it is working correctly. I'll need to tweak a few things and get back to you. By the way, thanks SO much for taking the time to help me!

Answer 1

Your first aproach only work because a bad design on MySQL.
The right aproach should be
SQL Fiddle Demo
SELECT a.id, a.authorId, a.answer, a.votes
FROM ( SELECT authorId,
MAX(votes) as votes
FROM answers
GROUP BY authorId ) AS h
JOIN answers a
ON a.authorId = h.authorId
AND a.votes = h.votes;
OUTPUT
| id | authorId | answer | votes |
|----|----------|--------|-------|
| 2 | a | x2 | 21 |
| 4 | b | x1 | 23 | ==>
| 5 | b | x2 | 23 | ==> duplicates max value are possible
But this have issue if several answer has same score. You need to include some logic to decide which one to show.
Also you can use variable to get the highest score.
SELECT *
FROM (
SELECT a.*,
#rownum := if(#prev_value = authorId,
#rownum + 1,
1) rn,
#prev_value := authorId as prev
FROM answers a,
(SELECT #rownum := 0, #prev_value := '') r
ORDER BY authorId, votes DESC
) T
WHERE T.rn = 1;
OUTPUT
| id | authorId | answer | votes | rn | prev |
|----|----------|--------|-------|----|------|
| 4 | b | x1 | 23 | 1 | b | => only one is show but would
| 2 | a | x2 | 21 | 1 | a | be random unless you specify some rule.
Now for your question you also need to use LEFT JOIN instead of JOIN to get the students without scores.
Something like this
SELECT au.authorId,
COALESCE(t.id, 0) as id,
COALESCE(t.answer , 0) as answer ,
COALESCE(t.votes , 0) as votes
FROM authors au
LEFT JOIN (
SELECT *
FROM (
SELECT a.*,
#rownum := if(#prev_value = authorId,
#rownum + 1,
1) rn,
#prev_value := authorId as prev
FROM answers a,
(SELECT #rownum := 0, #prev_value := '') r
ORDER BY authorId, votes DESC
) T
WHERE T.rn = 1) T
ON au.authorId = T.authorId
OUTPUT
| authorId | id | answer | votes |
|----------|----|--------|-------|
| a | 2 | x2 | 21 |
| b | 4 | x1 | 23 |
| c | 0 | 0 | 0 |