thinkphp联表查询这样写为什么没有结果？

Question

$sql = "select p.*,u.key_name from record p left join sys_user u on p.create_entity=u.id where u.key_name='admin' and p.punch_time between '2017-03-01' and '2017-03-08'";
$hr_punch_record = M() -> query("$sql");
var_dump($hr_punch_record);

sql语句放到SSMS里是可以查询出结果的，但是上面怎么为空呢？

Answer 1

$hr_punch_record = M() -> query("$sql");
换成
$hr_punch_record = M() -> query($sql);

Answer 2

M()应该要实例化数据表吧，M('Record')