NOT EXISTS not considered in the results - php

Question

I'm trying to get the results from 'comments' table with the exception of the ones in the 'readsbaby' table.
The result i'm getting is from comments but no effect of the NOT EXISTS statement, so the result is all the comments.
Both tables have common data that should not be included in the result.
I checked the data and the syntax many times.
Still this query will return all comments without taking in consideration the AND NOT EXISTS close.
public function get_user_comments($post_id)
{
$user_id = $this->session->userdata('id');
$group_id = $this->session->userdata('group_id');
$sql = "SELECT *
FROM comments
WHERE DATE(created_on) > DATE_SUB(CURDATE(), INTERVAL 1 DAY)
AND comments.group_id = " . $group_id . "
AND comments.user_id != " . $user_id . "
AND NOT EXISTS ( SELECT *
FROM readsbaby
WHERE comments.id = readsbaby.notification_id
AND comments.group_id = readsbaby.group_id
AND readsbaby.user_id = " . $this->session->userdata('id') . "
AND comments.nature1 = readsbaby.notification_type
) ";
return $data=$this->db->query($sql)->result_array();
}
Expecting to get the result filtered by the NOT EXISTS close.

Answer 1

I'd change the query this way, to have only the rows without any match in the table readsbaby:
public function get_user_comments($post_id)
{
$user_id = $this->session->userdata('id');
$group_id = $this->session->userdata('group_id');
$sql = "SELECT *
FROM comments
LEFT OUTER JOIN readsbaby ON comments.id = readsbaby.notification_id
AND comments.group_id = readsbaby.group_id
AND readsbaby.user_id = " . $this->session->userdata('id') . "
AND comments.nature1 = readsbaby.notification_type
WHERE DATE(created_on) > DATE_SUB(CURDATE(), INTERVAL 1 DAY)
AND comments.group_id = " . $group_id . "
AND comments.user_id != " . $user_id . "
AND ISNULL(readsbaby.notification_id )";
return $data=$this->db->query($sql)->result_array();
}

Answer 2

This is easier to accomplish using the correct mysql JOIN.
From your post, I have gathered that you want to retrieve all the values from comments where they don't exist in readsbaby. The id column in comments associates with the notification_id in readsbaby for any 'matching' (duplicate) entries.
On this assumption, you can do this at a simplistic level like so
SELECT c.* FROM comments c
LEFT JOIN readsbaby r ON c.id = r.id WHERE r.notification_id IS NULL;
...which should translate to your context as something like:
SELECT c.* FROM comments c
LEFT JOIN readsbaby r ON c.id = r.notification_id
WHERE
r.notification_id IS NULL
AND DATE(c.created_on) > DATE_SUB(CURDATE(), INTERVAL 1 DAY)
AND c.group_id = " . $group_id . "
AND c.user_id != " . $user_id . "
If, I have this wrong and you're looking to only get the data that isn't present in both, you'd need to use something like:
SELECT c.* FROM comments c
LEFT JOIN readsbaby r ON c.id = r.notification_id
WHERE
r.notification_id IS NULL
AND DATE(c.created_on) > DATE_SUB(CURDATE(), INTERVAL 1 DAY)
AND c.group_id = " . $group_id . "
AND c.user_id != " . $user_id . "
UNION
SELECT r.* FROM readsbaby r
LEFT JOIN comments c ON c.id = r.notification_id
WHERE
c.id IS NULL
AND r.group_id = " . $group_id . "
AND r.user_id != " . $user_id . "
This may need tweaking to suit, as there's very little to explain the data and table relationship.
Note: Do not inject SQL as you've done. Consider parameterising your query with PDO or any other prepared statements (e.g. http://php.net/manual/en/pdo.prepare.php)