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 | [e-mail]
2 | [e-mail]
3 | [e-mail]

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

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 );
    }
}