MySQL query not producing the correct result - mysql

Question

Given this:
MariaDB [master]> describe history;
+--------------+------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------------+------------------+------+-----+---------+----------------+
| historyid | int(10) unsigned | NO | PRI | NULL | auto_increment |
| amount | float | NO | | NULL | |
| subsidy | char(1) | NO | | NULL | |
| last_payment | date | NO | | NULL | |
| amount_paid | float | NO | | NULL | |
| balance | float | NO | | NULL | |
| attend | char(1) | NO | | N | |
| attend_date | date | NO | | NULL | |
| groupid | int(11) unsigned | NO | | NULL | |
| clientid | int(10) unsigned | NO | MUL | NULL | |
| memberid | int(10) unsigned | NO | MUL | NULL | |
+--------------+------------------+------+-----+---------+----------------+
MariaDB [master]> describe participation;
+-----------+------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-----------+------------------+------+-----+---------+----------------+
| partid | int(11) | NO | PRI | NULL | auto_increment |
| notes | varchar(255) | NO | | NULL | |
| groupdate | date | NO | | NULL | |
| clientid | int(10) unsigned | NO | MUL | NULL | |
| memberid | int(10) unsigned | NO | MUL | NULL | |
+-----------+------------------+------+-----+---------+----------------+
MariaDB [master]> select * from participation;
+--------+-----------+------------+----------+----------+
| partid | notes | groupdate | clientid | memberid |
+--------+-----------+------------+----------+----------+
| 194 | test test | 2016-01-26 | 3 | 1 |
+--------+-----------+------------+----------+----------+
How do I write the following query
MariaDB [master]> SELECT attend_date, groupdate, h.clientid, h.memberid
-> FROM history AS h
-> LEFT JOIN participation AS p ON groupdate = attend_date
-> WHERE h.memberid = "1"
-> AND MONTH(attend_date) = "1"
-> AND YEAR(attend_date) = "2016"
-> AND attend_date <> "0000-00-00"
-> ORDER BY attend_date ASC;
+-------------+------------+----------+----------+
| attend_date | groupdate | clientid | memberid |
+-------------+------------+----------+----------+
| 2016-01-26 | 2016-01-26 | 3 | 1 |
| 2016-01-26 | 2016-01-26 | 4 | 1 |
| 2016-01-26 | 2016-01-26 | 1 | 1 |
| 2016-01-26 | 2016-01-26 | 2 | 1 |
| 2016-01-28 | NULL | 3 | 1 |
| 2016-01-28 | NULL | 4 | 1 |
| 2016-01-28 | NULL | 1 | 1 |
| 2016-01-28 | NULL | 2 | 1 |
+-------------+------------+----------+----------+
so that my return is like this
+-------------+------------+----------+----------+
| attend_date | groupdate | clientid | memberid |
+-------------+------------+----------+----------+
| 2016-01-26 | 2016-01-26 | 3 | 1 |
| 2016-01-26 | NULL | 4 | 1 |
| 2016-01-26 | NULL | 1 | 1 |
| 2016-01-26 | NULL | 2 | 1 |
| 2016-01-28 | NULL | 3 | 1 |
| 2016-01-28 | NULL | 4 | 1 |
| 2016-01-28 | NULL | 1 | 1 |
| 2016-01-28 | NULL | 2 | 1 |
+-------------+------------+----------+----------+
The whole idea here is to be able to pull all the clients who don't yet have notes applied to a groupdate matching history table's attend_date. And only when a note is posted will the groupdate be set to the attend_date. Yet, the query, as I have it , returns a groupdate of 2016-01-26 for all four clients instead of just the one. Thanks in advance for your help.

Answer 1

Looking at your table structure, I see the relationship between your attend_date and groupdate on which you attempted to join, but I also see that those tables have clientid and memberid in common.
Since you only joined on the date, you're getting back a cartesian product of all clients and all dates for that condition. You can make the join's ON clause more specific by adding additional conditions with a logical AND.
LEFT JOIN participation AS p
ON groupdate = attend_date
AND h.clientid = p.clientid
It isn't fully obvious to me if your desired result set also requires a condition to match memberid between those tables, but if it does, the solution is as simple as another AND.
LEFT JOIN participation AS p
ON groupdate = attend_date
AND h.memberid = p.memberid
It helps not to think of a join's ON clause as merely a match between common columns, though that's the most common way it is used. Instead the ON clause just needs to present some condition that when true results in a joined row being returned (just like you're used to with a WHERE clause). That means you can place just about anything there which can be evaluated as true or false, allowing for complex joining logic. And the same can be said for ORDER BY and GROUP BY.