Here is what I'm trying to do. I have a table with user assessments which may contain duplicate rows. I'm looking to only get DISTINCT values for each user.
In the example of the table below. If only user_id 1 and 50 belongs to the specific location, then only the unique video_id's for each user should be returned as the COUNT. User 1 passed video 1, 2, and 1. So that should only be 2 records, and user 50 passed video 2. So the total for this location would be 3. I think I need to have two DISTINCT's in the query, but am not sure how to do this.
+-----+----------+----------+
| id | video_id | user_id |
+-----+----------+----------+
| 1 | 1 | 1 |
| 2 | 2 | 50 |
| 3 | 1 | 115 |
| 4 | 2 | 25 |
| 5 | 2 | 1 |
| 6 | 6 | 98 |
| 7 | 1 | 1 |
+-----+----------+----------+
This is what my current query looks like.
$stmt2 = $dbConn->prepare("SELECT COUNT(DISTINCT user_assessment.id)
FROM user_assessment
LEFT JOIN user ON user_assessment.user_id = user.id
WHERE user.location = '$location'");
$stmt2->execute();
$stmt2->bind_result($video_count);
$stmt2->fetch();
$stmt2->close();
So my query returns all of the count for that specific location, but it doesn't omit the non-unique results from each specific user.
Hope this makes sense, thanks for the help.
解决方案
The query below joins a sub-query that fetches the distinct videos per user. Then, the main query does a sum on those numbers to get the total of videos for the location.
SELECT
SUM(video_count)
FROM
user u
INNER JOIN
( SELECT
ua.user_id,
COUNT(DISTINCT video_id) as video_count
FROM
user_assessment ua
GROUP BY
ua.user_id) uav on uav.user_id = u.user_id
WHERE
u.location = '$location'
Note, that since you already use bindings, you can also pass $location in a bind parameter. I leave this to you, since it's not part of the question. ;-)
