如果字段为空，如何防止数据发送到数据库？

How would I go about not sending the data to the database if the some of the fields are left empty? Right as of now, if a field is empty on the form, the database is replacing whatever was in the field with blank data

UPDATE: Forgot to mention, it doesn't matter if the some of the fields are left blank, that should be allowed.

My code

<?php
if (isset($_POST['eventname'], $_POST['date'], $_POST['eventvenue'] , $_POST['eventtime'], $_POST['eventcost'])){

$eventname = ($_POST['eventname']);
$eventdate = ($_POST['date']);
$eventtime = ($_POST['eventtime']) . ":00";
$eventvenue = ($_POST['eventvenue']);
$eventcost = ($_POST['eventcost']);

$result = mysql_query("UPDATE event set event_name = '" . $eventname . "', event_date = '" . $eventdate . "', event_time = '" . $eventtime . "', event_venue = '" . $eventvenue ."', event_cost = '" . $eventcost ."'");
}
?>

Try some thing like This

$query= "UPDATE event set ":

If(isset($var1)){
$query.= " var1=".$var1;
}else if (isset($var2)){

$query.= " var2=".$var2;
}

and so forth and then

$result = mysql_query($query);

You can read on PHP's function empty()

empty() on PHP.net

Example usage:

if(empty($eventname))
{
    echo "You have not set event name";
} else {
    mysqli_query(...);
}

As said on comments, do not use the deprecated mysql_* functions, use either mysqli_* or PDO.

This is an example using prepared statements; it builds the update statement based on whether the field is empty (zero length) or not.

Afterwards, the prepared statement is executed.

$updates = [];
$parameters = [];

if (strlen($_POST['eventname'])) {
    $updates[] = 'event_name = ?';
    $parameters[] = $_POST['eventname'];
}

// ...

if (strlen($_POST['eventtime'])) {
    $updates[] = "event_time = ?";
    $parameters[] = $_POST[$field] . ':00';
}

if ($updates) {
    $sql = sprintf('UPDATE event SET %s WHERE xxx', join(',', $updates));
    $stmt = $db->prepare($sql);
    $stmt->execute($parameters);
}