Counting votes in a MySQL table only once or twice - mysql

Question

I've got the following table:
+-----------------+
| id| user | vote |
+-----------------+
| 1 | 1 | text |
| 2 | 1 | text2|
| 3 | 2 | text |
| 4 | 3 | text3|
| 5 | 2 | text |
+-----------------+
What I want to do is to count the "votes"
SELECT COUNT(vote), vote FROM table GROUP BY vote
That works fine. Output:
+-------------------+
| count(vote)| vote |
+-------------------+
| 3 | text |
| 1 | text2|
| 1 | text3|
+-------------------+
But now I only want to count the first or the first and the second vote from a user.
So result what I want is (if I count only the first vote):
+-------------------+
| count(vote)| vote |
+-------------------+
| 2 | text |
| 1 | text3|
+-------------------+
I tried to work with count(distinct...) but can get it work.
Any hint in the right direction?

Answer 1

You can do this in a single SQL statement with something like this:
SELECT vote, COUNT(vote)
FROM
(
SELECT MAX(user), vote
FROM table1
GROUP BY user
) d
GROUP BY vote
Note that this only gives you 1 vote not 1 or 2.

Answer 2

The easiest way would be to use one of the "row numbering" solutions listed in this SO question. Then your original query's almost there:
SELECT
COUNT(vote),
vote
FROM tableWithRowNumberAdded
WHERE MadeUpRowNumber IN (1,2)
GROUP BY vote
My alternative is much longer winded and calls for working tables. These can be "real" tables in your schema, or whatever flavour of intermediate resultsets you are comfortable with.
Start by getting the first vote for each user:
SELECT user, min(id) FROM table GROUP BY user
Put this in a working table; let's call it FirstVote. Next we can get each user's second vote, if any:
SELECT user, min(id) FROM table WHERE id not in (select id from FirstVote) GROUP BY user
Let's call the result of this SecondVote. UNION FirstVote to SecondVote, join this to the original table and group by vote. There's your answer!
SELECT
vote,
COUNT(*)
FROM table
INNER JOIN
(
SELECT id FROM FirstVote
UNION ALL
SELECT id FROM SecondVote
) as BothVotes
ON BothVotes.id = table.id
GROUP BY vote
Of course it could be structured as a single statement with multiple sub-queries but that would be horrendous to maintain, or read in this forum.

Answer 3

This is a very triky question for MySQL. On other systems there windowed functions: it performs a calculation across a set of table rows that are somehow related to the current row.
MySQL lacks this functionality. So one should look for a workaround. Here is the problem description and couple solutions suggested: MySQL and window functions.
I also assume that first 2 votes by the User can be determined by Id: earlier vote has smaller Id.
Based on this I would suggest this solution to your problem:
SELECT
Vote,
Count (*)
FROM
Table,
(
SELECT
user_id, SUBSTRING_INDEX(GROUP_CONCAT(Id ORDER BY user_id ASC), ',', 2) AS top_IDs_per_user
FROM
Table
GROUP BY
user_id
) s_top_IDs_per_User
WHERE
Table.user_id = s_top_IDs_per_User.User_id and
FIND_IN_SET(Id, s_top_IDs_per_User.top_IDs_per_user)
GROUP BY Vote
;