Question

I would like to add a value to each row that I get from my query depending on if a row exist in another table. Is there a smart way to achieve this?

This is the code I have:

$sth = mysql_query("SELECT tbl_subApp2Tag.*, tbl_tag.* FROM tbl_subApp2Tag LEFT JOIN tbl_tag ON tbl_subApp2Tag.tag_id = tbl_tag.id WHERE tbl_subApp2Tag.subApp_id = '".$sub."' ORDER BY tbl_tag.name ASC");
if(!$sth) echo "Error in query: ".mysql_error();
while($r = mysql_fetch_assoc($sth)) {
    $query = "SELECT * FROM tbl_userDevice2Tag WHERE tag_id='".$r['id']."' AND userDevice_id='".$user."'";
    $result = mysql_query($query) or die(mysql_error());
    if (mysql_num_rows($result)) {
        $r['relation'] = true;
        $rows[] = $r; //Add 'relation' => true to this row
    } else {
        $r['relation'] = false;
        $rows[] = $r; //Add 'relation' => false to this row
    }
}
print json_encode($rows);

Where the //Add … is, is where I would like to insert the extra value. Any suggestions of how I can do this?
I’m still a beginner in PHP so if there are anything else that I have missed please tell me.

EDIT: Second query was from the wrong table. This is the correct one.

Answer 1

Edited Edited below query to reflect new information because I don’t like leaving things half-done.

$sth = mysql_query("
    SELECT
        tbl_subApp2Tag.*,
        tbl_tag.*,
        ISNULL(tbl_userDevice2Tag.userDevice_id) AS relation

    FROM tbl_subApp2Tag
    LEFT JOIN tbl_tag
        ON tbl_tag.id = tbl_subApp2Tag.tag_id

    LEFT JOIN tbl_userDevice2Tag
        ON tbl_userDevice2Tag.tag_id = tbl_tag.id
        AND tbl_userDevice2Tag.userDevice_id = '".$user."'

    WHERE tbl_subApp2Tag.subApp_id = '".$sub."'
    ORDER BY tbl_tag.name ASC
");

Though the above feels like the LEFT JOIN on tbl_tag is the wrong way around, but it’s hard to tell as you are vague on your eventual aim. For example, if I was to assume the following

1. Tags will always exist
2. subApp2Tag will always exist
3. You want to know if a record in tbl_userDevice2Tag matches the above

Then I would do the following instead. The INNER JOIN means that it won’t worry about records in tbl_tag that are not on the requested subApp_id which in turn will limit the other joins.

$sth = mysql_query("
    SELECT
        tbl_subApp2Tag.*,
        tbl_tag.*,
        ISNULL(tbl_userDevice2Tag.userDevice_id) AS relation

    FROM tbl_tag

    INNER JOIN tbl_subApp2Tag
        ON tbl_subApp2Tag.tag_id = tbl_tag.id
        AND tbl_subApp2Tag.subApp_id = '".$sub."'

    LEFT JOIN tbl_userDevice2Tag
        ON tbl_userDevice2Tag.tag_id = tbl_tag.id
        AND tbl_userDevice2Tag.userDevice_id = '".$user."'

    ORDER BY tbl_tag.name ASC
");