LEFT JOIN not working as expected with sub-query - mysql

Question

I've got the SQL query below:
SELECT message, sent_date, user_id
FROM messages
LEFT JOIN numbers ON messages.from_id = numbers.id
It returns all the rows (about 4000) in the messages table with additional columns coming from the numbers table. So far, this is what I would expect.
Now I left join this sub-query to another table, again using a left join:
SELECT message, sent_date
FROM (
SELECT message, sent_date, user_id
FROM messages
LEFT JOIN numbers ON messages.from_id = numbers.id
) AS table1
LEFT JOIN users ON table1.user_id = users.id
However, it only returns about 200 rows so many are missing. Since this is a left join I would expect all the rows from table1 to be in the result. Can anybody see what the issue is?
Edit:
So for information here are the 3 relevant tables (with irrelevant columns removed):
CREATE TABLE IF NOT EXISTS `messages` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`message` text CHARACTER SET utf8 NOT NULL,
`from_id` int(11) DEFAULT NULL,
`sent_date` datetime NOT NULL,
PRIMARY KEY (`id`),
KEY `from_id` (`from_id`),
) ENGINE=InnoDB DEFAULT CHARSET=latin1 AUTO_INCREMENT=101553 ;
CREATE TABLE IF NOT EXISTS `numbers` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`user_id` int(11) DEFAULT NULL,
`number` varchar(32) NOT NULL,
PRIMARY KEY (`id`),
KEY `user_id` (`user_id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 AUTO_INCREMENT=6408 ;
CREATE TABLE IF NOT EXISTS `users` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(256) CHARACTER SET utf8 DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 AUTO_INCREMENT=2395 ;

Answer 1

You can try alternative method to debug the issue:
CREATE TEMPORARY table tmp1 AS SELECT message, sent_date, user_id
FROM messages
LEFT JOIN numbers
ON messages. from_id = numbers.id;
and then see whether this query works.
SELECT message, sent_date
FROM tmp1 table1
LEFT JOIN users
ON table1.user_id = users.id;
Also for your case make sure that there are no other insert or updates in between. otherwise use transactions.

Answer 2

table1 sometimes won't have a UserID - so that'll be null, so those results will be missing?

Answer 3

I don't have an exact answer to your question, but if I have to start thinking, I will first find out what 3800 rows are missing and try to see the pattern (is it because user_id are null or duplicate)
SELECT message, sent_date, user_id
FROM messages
LEFT JOIN numbers ON messages.from_id = numbers.id
MINUS
(SELECT table1.message, table1.sent_date, table1.user_id
FROM (
SELECT message, sent_date, user_id
FROM messages
LEFT JOIN numbers ON messages.from_id = numbers.id
) AS table1
LEFT JOIN users ON table1.user_id = users.id)

Answer 4

Try this, I think it's a scoping issue on user_id.
SELECT table1.message, table1.sent_date
FROM (
SELECT messages.message, messages.sent_date, numbers.user_id
FROM messages
LEFT JOIN numbers ON messages.from_id = numbers.id
) AS table1
LEFT JOIN users ON table1.user_id = users.id
I'm not sure if user_id is in messages or numbers.

Answer 5

There is no way this should happen.
Try this variation:
SELECT
m.message, m.sent_date, n.user_id
FROM
messages m
LEFT JOIN
numbers AS n ON m.from_id = n.id
LEFT JOIN
users AS u ON n.user_id = u.id ;