MySQL连续计算2列并与另一个查询联接

Question

I have a table visits like this:

+--------------+-------------+------+-----+---------------------+----------------+
| Field        | Type        | Null | Key | Default             | Extra          |
+--------------+-------------+------+-----+---------------------+----------------+
| id           | int(11)     | NO   | PRI | NULL                | auto_increment |
| vis_id       | int(11)     | NO   |     | NULL                |                |
| unit         | int(11)     | NO   |     | NULL                |                |
| time_in      | timestamp   | NO   |     | CURRENT_TIMESTAMP   |                |
| time_out     | timestamp   | NO   |     | 0000-00-00 00:00:00 |                |
| in_username  | varchar(16) | NO   |     | NULL                |                |
| out_username | varchar(16) | NO   |     | NULL                |                |
+--------------+-------------+------+-----+---------------------+----------------+

and a table users like this:

+------------+-------------+------+-----+---------------------+----------------+
| Field      | Type        | Null | Key | Default             | Extra          |
+------------+-------------+------+-----+---------------------+----------------+
| id         | int(11)     | NO   | PRI | NULL                | auto_increment |
| fname      | varchar(32) | NO   |     | NULL                |                |
| lname      | varchar(32) | NO   |     | NULL                |                |
| date_added | timestamp   | NO   |     | CURRENT_TIMESTAMP   |                |
| username   | varchar(16) | NO   |     | NULL                |                |
| password   | varchar(40) | NO   |     | NULL                |                |
| auth_level | int(1)      | NO   |     | 1                   |                |
| last_login | timestamp   | NO   |     | 0000-00-00 00:00:00 |                |
+------------+-------------+------+-----+---------------------+----------------+

I want to be able to count how many times each user is in in_username AND out_username ... The query I was using before looks like this:

select count(*) as "count", u.fname as "fname" 
    from visits v
        inner join users as u on u.username = v.in_username
    group by u.username order by u.fname

But that only returns how many in_username 's there are... I'd like to have both in the same query if possible, so I could get results like this:

+----------+-----------+----------+
| count_in | count_out | fname    |
+----------+-----------+----------+
|      118 |       224 | Bo       |
|       27 |        64 | James    |
|      147 |       138 | Jeremy   |
|       23 |        37 | Jim      |
|      182 |       172 | Robert   |
|      120 |       158 | Tom      |
+----------+-----------+----------+

Where count_in is how many times their username appears in visits.in_username , and count_out is how many times their username appears in visits.out_username

Everything I've tried with UNION seems to add the counts together, or removes rows for some reason. Any ideas?

Answer 1

Do a subquery to get each total, combine them with UNION , and then merge them with SUM() .

SELECT SUM(count_in) count_in, SUM(count_out) count_out, fname
FROM (SELECT COUNT(*) count_in, 0 count_out, in_username fname
      FROM visits v
      GROUP BY fname
      UNION
      SELECT 0 count_in, COUNT(*) count_out, out_username fname
      FROM visits v
      GROUP BY fname) combined