Question

I have the following table with messages:

+---------+---------+------------+----------+
| msg_id  | user_id | m_date     |  m_time  |
+-------------------+------------+----------+
|   1     | 1       | 2011-01-22 | 06:23:11 |
|   2     | 1       | 2011-01-23 | 16:17:03 |
|   3     | 1       | 2011-01-23 | 17:05:45 |
|   4     | 2       | 2011-01-22 | 23:58:13 |
|   5     | 2       | 2011-01-23 | 23:59:32 |
|   6     | 2       | 2011-01-24 | 21:02:41 |
|   7     | 3       | 2011-01-22 | 13:45:00 |
|   8     | 3       | 2011-01-23 | 13:22:34 |
|   9     | 3       | 2011-01-23 | 18:22:34 |
|  10     | 3       | 2011-01-24 | 02:22:22 |
|  11     | 3       | 2011-01-24 | 13:12:00 |
+---------+---------+------------+----------+

What I want is for each day, to see how many messages each user has sent BEFORE and AFTER 16:00:

SELECT 
    user_id, 
    m_date, 
    SUM(m_time <= '16:00') AS before16, 
    SUM(m_time > '16:00') AS after16 
FROM messages 
GROUP BY user_id, m_date
ORDER BY user_id, m_date ASC

This produces:

user_id m_date      before16  after16
-------------------------------------
1       2011-01-22  1         0
1       2011-01-23  0         2
2       2011-01-22  0         1
2       2011-01-23  0         1
2       2011-01-24  0         1
3       2011-01-22  1         0
3       2011-01-23  1         1
3       2011-01-24  2         0

Because user 1 has written no messages on 2011-01-24, this date is not in the resultset. However, this is undesirable. I have a second table in my database, called “date_range”:

+---------+------------+
| date_id | d_date     |
+---------+------------+
| 1       | 2011-01-21 |
| 1       | 2011-01-22 |
| 1       | 2011-01-23 |
| 1       | 2011-01-24 |
+---------+------------+

I want to check the “messages” against this table. For each user, all these dates have to be in the resultset. As you can see, none of the users have written messages on 2011-01-21, and as said, user 1 has no messages on 2011-01-24. The desired output of the query would be:

user_id d_date      before16  after16
-------------------------------------
1       2011-01-21  0         0
1       2011-01-22  1         0
1       2011-01-23  0         2
1       2011-01-24  0         0
2       2011-01-21  0         0
2       2011-01-22  0         1
2       2011-01-23  0         1
2       2011-01-24  0         1
3       2011-01-21  0         0
3       2011-01-22  1         0
3       2011-01-23  1         1
3       2011-01-24  2         0

How can I link the two tables so that the query result also holds rows with zero values for before16 and after16?

Edit: yes, I have a “users” table:

+---------+------------+
| user_id | user_date  |
+---------+------------+
| 1       | foo        |
| 2       | bar        |
| 3       | foobar     |
+---------+------------+

Answer 1

Test bed:

create table messages (msg_id integer, user_id integer, _date date, _time time);
create table date_range (date_id integer, _date date);
insert into messages values
       (1,1,'2011-01-22','06:23:11'),
       (2,1,'2011-01-23','16:17:03'),
       (3,1,'2011-01-23','17:05:05');
insert into date_range values
       (1, '2011-01-21'),
       (1, '2011-01-22'),
       (1, '2011-01-23'),
       (1, '2011-01-24');

Query:

SELECT p._date, p.user_id,
       coalesce(m.before16, 0) b16, coalesce(m.after16, 0) a16
  FROM
      (SELECT DISTINCT user_id, dr._date FROM messages m, date_range dr) p
  LEFT JOIN
      (SELECT user_id, _date,
              SUM(_time <= '16:00') AS before16,
              SUM(_time > '16:00') AS after16 
         FROM messages 
        GROUP BY user_id, _date
        ORDER BY user_id, _date ASC) m
    ON p.user_id = m.user_id AND p._date = m._date;

EDIT:

1. Your initial query is left as is, I hope it doesn’t requires any explanations;

2. SELECT DISTINCT user_id, dr._date FROM messages m, date_range dr will return a cartesian or CROSS JOIN of two tables, which will give me all required date range for each user in subject. As I’m interested in each pair only once, I use DISTINCT clause. Try this query with and without it;

3. Then I use LEFT JOIN on two sub-selects.

   This join means: first, INNER join is performed, i.e. all rows with matching fields in the ON condition are returned. Then, for each row in the left-side relation of the join that has no matches on the right side, return NULLs (thus the name, LEFT JOIN, i.e. left relation is always there and right is expected to have NULLs). This join will do what you expect — return user_id + date combinations even if there were no messages in the given date for a given user. Note that I use user_id + date sub-select first (on the left) and messages query second (on the right);

4. coalesce() is used to replace NULL with zero.

I hope this clarifies how this query works.