mysqli 为什么不提示字段错误

$mysqli = new mysqli("localhost", "my_user", "my_password", "world");

/* check connection */
if (mysqli_connect_errno()) {
    printf("Connect failed: %s\n", mysqli_connect_error());
    exit();
}

//  $mysqli->affected_rows 取得前一次 MySQLI 操作所影响的记录行数
$mysqli->query("update {$tpre}member set ttid='2000'  where userid123='10000'");
printf("Affected rows (UPDATE): %d\n", $mysqli->affected_rows);

<?php
$mysqli = new mysqli("localhost", "my_user", "my_password", "world");

/* check connection */
if ($mysqli->connect_errno) {
    printf("Connect failed: %s\n", $mysqli->connect_error); //mysql连接错误抛出
    exit();
}

if (!$mysqli->query("SET a=1")) {
    printf("Errormessage: %s\n", $mysqli->error); //mysql查询错误抛出
}
?>

<?php
$link = mysqli_connect("localhost", "my_user", "my_password", "world");

/* check connection */
if (mysqli_connect_errno()) {
    printf("Connect failed: %s\n", mysqli_connect_error());
    exit();
}

if (!mysqli_query($link, "SET a=1")) {
    printf("Errormessage: %s\n", mysqli_error($link));
}

?>