mySQL complex SELECT query? - php

Question

Here is my table structure:
users:
id | email
emails:
id | subject | body
user_emails:
user_id | email_id
It's very simple design. However, I need to select the first (lowest) emails.id for all users.id's that are not associated in user_emails.
To illustrate:
users:
id | email
1 | email#domain.com
2 | test#lol.com
3 | user#test.com
emails:
id | subject | body
1 | sub1 | body1
2 | sub2 | body2
user_emails:
user_id | email_id
1 | 1
1 | 2
2 | 1
As the data shows:
User 1 received email 1 and 2 already so the select should exclude
that user.
User 2 only received email 1, so it should select user 2
and email 2.
User 3 didn't receive any emails, so it should select user 3
and email 1.
I'll be doing this in PHP so any logic should be in written in PHP if it can't be done with mySQL alone.
Thanks in advance
EDIT: I should probably mention that there will be thousands of user ID's and hundreds of email id's. The script that will send out all these emails will run once daily. So, this should be as optimized as possible.

Answer 1

Could try the below:
SELECT * from users u
LEFT JOIN users_emails e ON u.id=e.user_id
LEFT OUTER JOIN emails m on m.id=1
WHERE e.user_id is NULL
Or use the below if the id in emails not necessary to be 1 as above query.
SELECT * from users u
LEFT JOIN users_emails e ON u.id=e.user_id
LEFT OUTER join emails m on m.id=(SELECT id from emails order by id limit 1)
WHERE e.user_id is NULL

Answer 2

I thought of a much simpler solution to all this. I added a lastSent column to the users table.
// Load all emails into an array. Use the id as the key.
$sql = "SELECT * FROM emails";
$query = mysql_query( $sql );
while( $row = mysql_fetch_assoc( $query ) ) {
$emails[ $row['id'] ] = $row;
}
// Load all users into an array.
$sql = "SELECT id, email, lastSent FROM users";
$query = mysql_query( $sql );
while( $row = mysql_fetch_assoc( $query ) ) {
$users = $row;
}
// Loop through each user and send next email from the list based on lastSent value.
foreach ( $users as $user ) {
$id = $user['lastSent'] + 1;
$email = $user['email'];
$subject = $emails[$id]['subject'];
$body = $emails[$id]['body'];
$send = mail($email, $subject, $body);
if ( $send ) {
// Update lastSent value.
$sql = "UPDATE users SET lastSent='$id' WHERE id='" . $user['id'] . "'";
mysql_query( $sql );
}
}

Answer 3

Give a try with the below query
select t.id, min(t.q_id) from (SELECT u.id, e.id as q_id FROM users u CROSS JOIN emails e) as t where (t.id, t.q_id) not in (select * from user_emails) group by t.id