注意：未定义的索引：第13行的C：\ wamp64 \ www \ quiz emove epeated.php中的Ans

Question

How to resolve this?

Notice: Undefined index: Ans in C:\wamp64\www\quizemoveepeated.php on line 13

My code:

require_once '../class.user.php';
$user_home = new USER();
$lstmtf = $user_home->runQuery("SELECT COUNT(Ans)
FROM answer AS a
LEFT JOIN students_records AS s ON a.Sr = s.Sr
WHERE s.Sr IS NULL");
    $lstmtf->execute();
$reg_rst = $lstmtf->fetch(PDO::FETCH_ASSOC);
$registered= $reg_rst['Ans'];
        echo $registered;

My table answer has column name Ans.

Actually, I want to count the numbers of rows which do not have values in students_records.

For Eg:

students_records

+----+-----+-----+ 
| Sr | SRN | ARN |
+----+-----+-----+
| 1  | ge  | aj  |
| 2  | ge  | bd  |
+----+-----+-----+  

answer

+----+-----+-----+
| Sr | SRN | ARN |
+----+-----+-----+
| 1  | ge  | aj  |
| 2  | ge  | aj  |
| 3  | ge  | ne  |
| 4  | ge  | bd  |
+----+-----+-----+ 

Here count should be 1. As the value "ne" in column ARN of table answer is no where in the rows of column ARN in table students_records.

Answer 1

You can get the desire output by following

SELECT count(a.Ans) as ans
FROM answer AS a
LEFT JOIN students_records AS s ON a.ARN = s.ARN
WHERE   s.ARN IS NULL

Here is working example