Select distinct from a list based on set of parameters - mysql

Question

I'm trying to get a distinct list of results, distinct based on user, where the selected result would be based on a set of parameters. To break it down, I have users, logs, and files. Each user can be on multiple logs and can have multiple files. Files CAN be associated with logs or not, and can also have a 'billing' flag set to true. What I'm trying to do when someone selects a log is bring up the list of files most closely associated with both the 'billing' flag and the log.
If the user has a file that is associated with the log AND has the
'billing' flag set to true, that is the result for that user.
If that is not available, the next would be the file that only has the 'billing' flag set to true (associated with any highest log or none).
If that is not available, the highest log number.
Here is the generalization of the tables:
Test Table:
+----+------+-----+
| ID | user | log |
+----+------+-----+
| 1 | 1 | 2 |
| 2 | 1 | 2 |
| 3 | 2 | 2 |
| 4 | 3 | 2 |
| 5 | 3 | 2 |
| 6 | 4 | 2 |
+----+------+-----+
File Table:
+----+-------+-----+---------+------+
| ID | file | log | billing | user |
+----+-------+-----+---------+------+
| 1 | a.pdf | 2 | 0 | 1 |
| 2 | b.pdf | 3 | 1 | 1 |
| 3 | c.pdf | 1 | 0 | 2 |
| 4 | d.pdf | 2 | 1 | 2 |
| 5 | e.pdf | 1 | 0 | 3 |
| 6 | f.pdf | 3 | 0 | 3 |
| 7 | g.pdf | 0 | 1 | 4 |
| 8 | h.pdf | 1 | 0 | 4 |
| 9 | i.pdf | 2 | 1 | 4 |
| 10 | j.pdf | 3 | 0 | 4 |
+----+-------+-----+---------+------+
In this case I would want to get:
+------+-------+-----+---------+
| user | file | log | billing |
+------+-------+-----+---------+
| 1 | b.pdf | 3 | 1 |
| 2 | d.pdf | 2 | 1 |
| 3 | f.pdf | 3 | 0 |
| 4 | i.pdf | 2 | 1 |
+------+-------+-----+---------+
My simplified query so far returns all files for the users but I'm having trouble grouping based on the above parameters.
SELECT
user,
file,
log,
billing
FROM
files
WHERE
user IN (
SELECT
DISTINCT(user)
FROM
tests
WHERE
log = 2
)
ORDER BY
CASE
WHEN log = 2 AND billing = 1 THEN 1
WHEN billing = 1 THEN 2
ELSE -1
END
Any help would be greatly appreciated.

Answer 1

You can use a separate query to get the results based on each of the 3 criteria specified in the OP, then UNION the results from these queries and fetch result from first query if available, otherwise from second query, otherwise from third query:
SELECT user, file, log, billing
FROM (
SELECT #row_number:=CASE WHEN #user=user THEN #row_number+1
ELSE 1
END AS row_number,
#user:=user AS user,
file, log, billing
FROM (
-- 1st query: has biggest priority
SELECT 1 AS pri, t.user, f.file, f.log, f.billing
FROM (SELECT DISTINCT user, log
FROM tests
WHERE log = 2) AS t
INNER JOIN files AS f
ON (t.user = f.user AND t.log = f.log AND f.billing = 1)
UNION ALL
-- 2nd query: priority = 2
SELECT 2 AS pri, t.user, f.file, f.log, f.billing
FROM (SELECT DISTINCT user, log
FROM tests
WHERE log = 2) AS t
INNER JOIN files AS f
ON (t.user = f.user AND f.billing = 1)
WHERE f.log > t.log OR f.log = 0
UNION ALL
-- 3rd query: priority = 3
SELECT 3 AS pri, t.user, f.file, f.log, f.billing
FROM (SELECT DISTINCT user, log
FROM tests
WHERE log = 2) AS t
INNER JOIN files AS f ON (t.user = f.user)
ORDER BY user, pri, log DESC ) s ) r
WHERE r.row_number = 1
ORDER BY user
pri column is used so as to discern and prioritize results between the three separate queries. #row_number and #user variables are used in order to simulate ROW_NUMBER() OVER (PARTITION BY user ORDER BY pri) window function. Using #row_number in the outermost query we can select the required record, i.e. the record having the highest priority within each 'user' partition.
SQL Fiddle Demo