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问题:

This is my form:

<!DOCTYPE html>
<html>
  <head>
  </head>
  <body>
    <form action="register_ajax.php" method="get">
    <input type="text" name="email">
    <input type="submit" value="test">
    </form>
  </body>
</html>

This is my php code:

<?php
    $dbc = mysqli_connect("localhost","root","*******","continental_tourism") OR die(mysqli_connect_error());
    $email = $_GET['email'];
    $query = "SELECT email FROM customer_info WHERE email = '$email' ";
    $r = mysqli_query($dbc, $query) OR die(mysqli_error($dbc));
    if($r)
        echo "Email address exists!";
    else
        echo "sss";
?>

If I enter a correct(Existing email on db) $r is true. But if I enter non existing email, then also $r is true. Why is that? Basically I want to detect the empty set. How can I do it?

Thanks!

回答1:

$r will only be false if there was SQL error. Otherwise it will always return a resource ID even if no rows are returned by your SELECT statement.

Use mysqli_num_rows() to count how many rows are returned. Zero means no one is using that email address.

if(mysqli_num_rows($r))
    echo "Email address exists!";
else
    echo "sss";

回答2:

mysqli_query() will always returns a MySQLi result resource if the query was successful. A resource will always evaluate to TRUE when tested as a boolean.

A successful query is not necessarily one that returns results, it is simply a query that did not fail. Queries are considered to have failed when there is an error, for example a syntax error or a processing error within the MySQL server - which case they will return FALSE. But a query that finds no results is still a successful query.

You need to use mysqli_num_rows() to determine whether the query found any results:

<?php
    $dbc = mysqli_connect("localhost","root","longhorn","continental_tourism") OR die(mysqli_connect_error());

    $email = $_GET['email'];

    $query = "SELECT email FROM customer_info WHERE email = '$email' ";

    $r = mysqli_query($dbc, $query) OR die(mysqli_error($dbc));

    if(mysqli_num_rows($r) > 0)
        echo "Email address exists!";
    else
        echo "sss"; 
?>

回答3:

mysqli_query will return a result object on success and false on failure. No rows selected is not considered failure. You can use mysqli_num_rows to check the number of rows returned.

回答4:

One needs to test to see if $r is true, and if row count is greater than 0.

The following works:

if ($r and mysqli_num_rows($r) > 0) {

  echo "rows exist"; } else {

echo "rows don't exist";  }

mysqli_query() always return true

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mysqli_query() always return true

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