在插入数据之前检查现有条目

Question

I am trying to insert new clients only if they don't already exist. I am having problems I think with the SQL statement, but I wanted to give context in the PHP too:

   $sql = "INSERT INTO clients
        (`studentEmail`, `studentPassword`, `parentEmail`, `parentPassword`,
        `studentFirstName`, `studentLastName`, `studentPhone`, `parentFirstName`,
        `parentLastName`, `parentPhone`, `school`)
         VALUES ('$studentEmail', '$studentPassword', '$parentEmail',
        '$parentPassword', '$studentFirstName', '$studentLastName',
        '$studentPhone', '$parentFirstName', '$parentLastName', '$parentPhone', '$school')
        SELECT studentEmail
        FROM clients
        WHERE not exists (select * from clients where studentEmail == '"$studentEmail"')";

Answer 1

You should check whether the email exists before inserting the data.

$check = mysql_query("SELECT studentEmail FROM clients WHERE  studentEmail = '{$studentEmail}';");

if (mysql_num_rows($check) == 0) {
    // insert
}