Question

I have a table like this;

+----+---------+-------------+
| id | user_id | screenWidth |
+----+---------+-------------+
|  1 |       1 |        1366 |
|  2 |       1 |        1366 |
|  3 |       1 |        1366 |
|  4 |       1 |        1366 |
|  5 |       2 |        1920 |
|  6 |       2 |        1920 |
|  7 |       3 |        1920 |
|  8 |       4 |        1280 |
|  9 |       5 |        1280 |
| 10 |       6 |        1280 |
+----+---------+-------------+

Along with loads of other data. This could be normalised if needed, originally I didn’t think I’d need to, but perhaps I should. Anyway,

I’d like a query that only counts the screenWidth values once per user, so the output would look like:

+-------------+-------+
| screenWidth | count |
+-------------+-------+
|        1366 |     1 |
|        1920 |     2 |
|        1280 |     3 |
+-------------+-------+

Rather than counting 1366 as 4 – this would avoid heavy users from skewing the data.

Is there a way to write a query to do this?

Answer 1

You have to get the DISTINCT count of users per screenwidth and here is the sample query that will fetch the results.

Click here to view the demo in SQL Fiddle

Script:

CREATE TABLE screenwidth 
(
    id INT NOT NULL
  , user_id INT NOT NULL
  , screenwidth INT NOT NULL
);

INSERT INTO screenwidth (id, user_id, screenwidth) VALUES
  (1, 1, 1366),
  (2, 1, 1366),
  (3, 1, 1366),
  (4, 1, 1366),
  (5, 2, 1920),
  (6, 2, 1920),
  (7, 3, 1920),
  (8, 4, 1280),
  (9, 5, 1280),
  (10, 6, 1280);

SELECT      screenwidth
        ,   COUNT(DISTINCT user_id) AS screenwidthcount
FROM        screenwidth
GROUP BY    screenwidth
ORDER BY    screenwidthcount;

Output:

SCREENWIDTH SCREENWIDTHCOUNT
----------- ----------------
1366               1
1920               2
1280               3