### count
获取数据表中的行数
count($table, $where)
* ##### table [string]
表名.
* ##### where (optional) [array]
WHERE 条件.
count($table, $join, $column, $where)
* ##### table [string]
表名.
* ##### join [array]
多表查询.
* ##### column [string]
需要统计的字段.
* ##### where (optional) [array]
WHERE 条件.
Return: [number] 行的数量.
>返回的是数字类型.
~~~
$count = $database->count("account", [
"gender" => "female"
]);
echo "We have " . $count . " female users.";
~~~
