mysql 查询某个字段不为空（附thinkphp写法）

1.如果要写某个字段不为空，mysql的表达式应为：

假设字段为：door_open_api_key，

那么：door_open_api_key is  not null AND door_open_api_key != ""

2. thinkphp中应写为

        $Reserve_system = M('Reserve_system');
        $where = array();
        $where['door_open_api_key'] = array('exp',' is not null AND door_open_api_key != ""');
        $reserve_systems = $Reserve_system->where($where)->select();