计算MySQL数据库中的记录的条数的命令是什么来着？

Question

select count(0) from tablename

Answer 1

select count(*) as Num from tablename

Answer 2

<?php
mysql_connect("localhost","user","password");
mysql_select_db("database");
$sql = "SELECT count(*) AS Num FROM `tablename`";
$res = mysql_query($sql);
$row = mysql_fetch_assoc($res);
echo $row['Num'];
?>

Answer 3

<?$sql="select count(0) from tablename";
$result=mysql_query($sql);
$total=mysql_result($result,0);?>

Answer 4

$sql= "SELECT * FROM tablename";
$result =  mysql_query ($sql, $connection);$total=mysql_num_rows($result);