Hello everyone! This is my first post, been a long time reader though.
First of all, it's been a while since I don't program and I'm rusty (very).

The problem is: I just can't learn how to use the JOIN or UNION commands, everything I read is confusing.
I'm simply trying to retrieve info from two different tables (same db) at the same time.

Here's my code:

if ($_POST['Teachers'] && $_POST['Students']) {
    $sql = "SELECT email FROM teachers"; //Here I also want to pull this off: SELECT email FROM students.
    $result = mysql_query($sql,$db);
    while ($row = mysql_fetch_row($result) or die(mysql_error())) {
        $mails.= "$row[0],";
    }
}

The $_POST variables come from a previous page where the school's principal will select if to send a mass mail to students and teachers or just one of them, so there are other if statements above, all working good. The $mails will be used on mail().

So, that's it, I'm trying my best and I've been reading for 3 days to no avail.
Thanks in advance guys.
Carlos I. Romero. (Systems Engineer and Administration: studying in Mexico).

This is a great opportunity to refactor the tables so that a student is a person and some things about being a student; a teacher is a person and some things about being a teacher. Then the select is done on the person table and your problem disappears.
like:

create table person (
  id integer not null auto_increment,
  email varchar(100)
  /* etc */
)
create table student (
  id integer not null auto_increment /* not strictly needed */,
  person_id integer not null foreign key references person.id, /* syntax? */
  current_GPA float /* or whatever a teacher would not have */
  /* etc */
)
 /* similar for teacher */

Hi! Thank you very much for helping me, really appreciate it.
So here's the code, depending on the election of the principal it will retrieve the requested mails (hopefully):

(Admins are teachers with privileges)

//Only Students
if ($_POST['Students'] empty($_POST['Teachers'] empty($_POST['Admins']) {
    $sql = "SELECT email FROM students ORDER BY nombre";
    $result = mysql_query($sql,$db);
    while ($row = mysql_fetch_row($result)) {
        $correos .= "$row[0],";
    }
//Only Teachers
} elseif ($_POST['Teachers'] && empty($_POST['Students']) empty($_POST['Admins']) {
    $sql = "SELECT email FROM teachers ORDER BY email";
    $result = mysql_query($sql,$db);
    while ($row = mysql_fetch_row($result)) {
        $correos .= "$row[0],";
    }
//Only Admins
} elseif ($_POST['Admins'] && empty($_POST['Students']) && empty($_POST['Teachers']) {
    $sql = "(SELECT email FROM teachers WHERE Admin='1') UNION (SELECT email FROM students) order by email";
    $result = mysql_query($sql,$db);
    while ($row = mysql_fetch_row($result)) {
        $correos .= "$row[0],";
    }
//Students and Teachers
} elseif ($_POST['Students'] && $_POST['Teachers'] && empty($_POST['Admins']) {
    $sql = "(SELECT email FROM students) UNION (SELECT email FROM teachers) order by email";
    $result = mysql_query($sql,$db);
    while ($row = mysql_fetch_row($result)) {
        $correos .= "$row[0],";
    }
//Students and Admins
} elseif ($_POST['Students'] && $_POST['Admins'] && empty($_POST['Teachers']) {
    $sql = "(SELECT email FROM alumnos) UNION (SELECT email FROM maestros WHERE Admin='1') order by email";
    $result = mysql_query($sql,$db);
    while ($row = mysql_fetch_row($result)) {
        $correos .= "$row[0],";
    }
//Teachers and Students
} elseif ($_POST['Teachers'] && $_POST['Students'] && empty($_POST['Admins']) {
    $sql = "(SELECT email FROM teachers) UNION (SELECT email FROM students) order by email";
    $result = mysql_query($sql,$db);
    while ($row = mysql_fetch_row($result)) {
        $correos .= "$row[0],";
    }
//Teachers and Admins
} elseif ($_POST['Teachers'] && $_POST['Admins'] && empty($_POST['Students']) {
    $sql = "SELECT email FROM maestros order by email";
    $result = mysql_query($sql,$db);
    while ($row = mysql_fetch_row($result)) {
        $correos .= "$row[0],";
    }
//Admins and students
} elseif ($_POST['Admins'] && $_POST['Students'] && empty($_POST['Teachers']) {
    $sql = "(SELECT email FROM teachers WHERE Admin='1') UNION (SELECT email FROM students) order by email";
    $result = mysql_query($sql,$db);
    while ($row = mysql_fetch_row($result)) {
        $correos .= "$row[0],";
    }
//Admins and Teachers
} elseif ($_POST['Admins'] && $_POST['Teachers'] && empty($_POST['Students']) {
    $sql = "(SELECT email FROM teachers WHERE Admin='1') UNION (SELECT email FROM teachers) order by email";
    $result = mysql_query($sql,$db);
    while ($row = mysql_fetch_row($result)) {
        $correos .= "$row[0],";
    }    
}

(Sorry for the double post, don't know what I did)
Hi! Thank you very much for helping me, really appreciate it.
So here's the code, depending on the election of the principal it will retrieve the requested mails (hopefully):

(Admins are teachers with privileges)

//Only Students
if ($_POST['Students'] empty($_POST['Teachers'] empty($_POST['Admins']) {
    $sql = "SELECT email FROM students ORDER BY name";
    $result = mysql_query($sql,$db);
    while ($row = mysql_fetch_row($result)) {
        $mails .= "$row[0],";
    }
//Only Teachers
} elseif ($_POST['Teachers'] && empty($_POST['Students']) empty($_POST['Admins']) {
    $sql = "SELECT email FROM teachers ORDER BY email";
    $result = mysql_query($sql,$db);
    while ($row = mysql_fetch_row($result)) {
        $mails .= "$row[0],";
    }
//Only Admins
} elseif ($_POST['Admins'] && empty($_POST['Students']) && empty($_POST['Teachers']) {
    $sql = "(SELECT email FROM teachers WHERE Admin='1') UNION (SELECT email FROM students) order by email";
    $result = mysql_query($sql,$db);
    while ($row = mysql_fetch_row($result)) {
        $mails .= "$row[0],";
    }
//Students and Teachers
} elseif ($_POST['Students'] && $_POST['Teachers'] && empty($_POST['Admins']) {
    $sql = "(SELECT email FROM students) UNION (SELECT email FROM teachers) order by email";
    $result = mysql_query($sql,$db);
    while ($row = mysql_fetch_row($result)) {
        $mails .= "$row[0],";
    }
//Students and Admins
} elseif ($_POST['Students'] && $_POST['Admins'] && empty($_POST['Teachers']) {
    $sql = "(SELECT email FROM alumnos) UNION (SELECT email FROM maestros WHERE Admin='1') order by email";
    $result = mysql_query($sql,$db);
    while ($row = mysql_fetch_row($result)) {
        $mails .= "$row[0],";
    }
//Teachers and Students
} elseif ($_POST['Teachers'] && $_POST['Students'] && empty($_POST['Admins']) {
    $sql = "(SELECT email FROM teachers) UNION (SELECT email FROM students) order by email";
    $result = mysql_query($sql,$db);
    while ($row = mysql_fetch_row($result)) {
        $mails .= "$row[0],";
    }
//Teachers and Admins
} elseif ($_POST['Teachers'] && $_POST['Admins'] && empty($_POST['Students']) {
    $sql = "SELECT email FROM maestros order by email";
    $result = mysql_query($sql,$db);
    while ($row = mysql_fetch_row($result)) {
        $mails .= "$row[0],";
    }
//Admins and students
} elseif ($_POST['Admins'] && $_POST['Students'] && empty($_POST['Teachers']) {
    $sql = "(SELECT email FROM teachers WHERE Admin='1') UNION (SELECT email FROM students) order by email";
    $result = mysql_query($sql,$db);
    while ($row = mysql_fetch_row($result)) {
        $mails .= "$row[0],";
    }
//Admins and Teachers
} elseif ($_POST['Admins'] && $_POST['Teachers'] && empty($_POST['Students']) {
    $sql = "(SELECT email FROM teachers WHERE Admin='1') UNION (SELECT email FROM teachers) order by email";
    $result = mysql_query($sql,$db);
    while ($row = mysql_fetch_row($result)) {
        $mails .= "$row[0],";
    }    
}

Thank you very much for your time!
Carlos I. Romero.

Code at line 17 does not match the comment at line 15 or the test at line 16

Code at line 59 is inefficient (double select not needed)

Note: Some of the table names are inconsistent: Translated or not.

You are welcome. Please mark the thread solved when you are happy that it is indeed done.

Thank you very much for your time and your help!
I fixed all the code, including the errors you pointed.
It is working now!
Thanks again!

Thank you very much for your time and your help!
I fixed all the code, including the errors you pointed.
It is working now!
Thanks again!