Sending message back if table row has value - php

Question

For example, have a query like this:
<?php
$sql = "INSERT INTO oppgave
(username, besvarelse, modulid)
VALUES
('$username', '$besvarelse', '$modulid');
";
mysql_query($sql, $tilkobling);
?>
How can I do this:
If modulid row is not empty, echo "you have sent this modul before";

Answer 1

Try this:
<?php
$sql = "INSERT INTO oppgave(username, besvarelse, modulid)
select '$username', '$besvarelse', '$modulid'
from dual
where not exists (select modulid from oppgave where modulid='$modulid')";
$result=mysql_query($sql, $tilkobling);
if(mysql_affected_rows()==0){
echo "Failed to insert duplicate modulid";
}else{
echo "inserted";
}
?>
In this SQL Query, it will allow insert only if $modulid isn't already exist in database table. Such situation you can also handle by applying UNIQUE constraint to table column (but remember, it allows NULL value at once)

Answer 2

Best way to do this, is by not allowing duplicates.You can do this with your SQL QUERY.Build your query by this:
$sqlString ="INSERT INTO oppgave('" . $username . "', '" . $besvarelse . "', '" . $modulid . "')
SELECT username, besvarelse, modulid
FROM oppgave
WHERE NOT EXISTS (SELECT *
FROM oppgave
WHERE modulid IS NULL)";
Please check all names and tablenames first ;)
Hope this helps.If this the answer, please mark as answer.