### max
获得数据表中，值最大的
max($table, $column, $where)
* ##### table [string]
表名.
* ##### column [string]
查询的字段列.
* ##### where (optional) [array]
WHERE 条件.
max($table, $join, $column, $where)
* ##### table [string]
表名.
* ##### join [array]
多表查询.
* ##### column [string]
字段名.
* ##### where (optional) [array]
条件.
Return: [number] 返回最大的值.
>这个返回数值类型.
~~~
$max = $database->max("account", "age", [
"gender" => "female"
]);
echo "The age of oldest female user is " . $max;
~~~
