怎样将occupied-room默认值设为一:SELECT `h`.`name` AS `hotel_name`, `rt`.`name` AS `room_type`, `rt`.`room_num` AS `total_rooms`, `r`.`booked_quantity` AS `occupied_rooms`,( `rt`.`room_num` - `r`.`booked_quantity` ) AS `vacant_rooms`,(( `r`.`booked_quantity` / `rt`.`room_num` ) * 100 ) AS `occupancy_rate%` FROM (( `hotels` `h` JOIN `room_types` `rt` ON (( `h`.`id` = `rt`.`hotel_id` ))) LEFT JOIN `has_bookingrel` `r` ON (( `rt`.`id` = `r`.`room_type_id` )))

您可以使用IFNULL函数将`occupied_rooms`的默认值设为1，如下所示：

SELECT
    `h`.`name` AS `hotel_name`,
    `rt`.`name` AS `room_type`,
    `rt`.`room_num` AS `total_rooms`,
    IFNULL(`r`.`booked_quantity`, 1) AS `occupied_rooms`,
    (`rt`.`room_num` - IFNULL(`r`.`booked_quantity`, 1)) AS `vacant_rooms`,
    ((IFNULL(`r`.`booked_quantity`, 1) / `rt`.`room_num`) * 100) AS `occupancy_rate%`
FROM
    `hotels` `h`
    JOIN `room_types` `rt` ON `h`.`id` = `rt`.`hotel_id`
    LEFT JOIN `has_bookingrel` `r` ON `rt`.`id` = `r`.`room_type_id`

这里使用了`IFNULL()`函数，如果`booked_quantity`为null，则将其默认值设为1。

相关问题

#256个block memory_size = 256 #pid进程号 class Process: def __init__(self, pid, block, duration): self.__block = block self.__duration = duration self.__pid = pid self.__memory = None @property def pid(self): return self.__pid @property def block(self): return self.__block @property def duration(self): return self.__duration def set_memory(self, memory_start, memory_end): self.__memory = (memory_start, memory_end) def get_memory(self): return self.__memory class MemoryAllocator: def __init__(self, memory_size): self.__memory_blocks = [None] * memory_size def memory_view(self): '''return the array of the use of memory blocks.''' return tuple(self.__memory_blocks) def allocate_memory(self, block_start, length, process): for block_id in range(block_start, block_start+length): assert self.__memory_blocks[block_id] is None, 'tend to allocate occupied blocks' self.__memory_blocks[block_id] = process process.set_memory(block_start, length) def free_memory(self, process): assert process.get_memory() is not None, 'process should already hold memory blocks' block_start, length = process.get_memory() for block_id in range(block_start, block_start+length): assert self.__memory_blocks[block_id] == process, 'the orresponding memory blocks should be assigned to the process' self.__memory_blocks[block_id] = None

这段代码实现了一个简单的内存分配器和进程类。其中，内存分配器使用一个数组来表示内存块的使用情况，每个进程对象都包含一个指向内存块的引用。

这段代码的实现还可以进行一些改进：

1. 内存分配器可以使用更高效的数据结构，比如位图，来表示内存块的使用情况。

2. 进程类可以添加一个方法来释放内存，而不是在内存分配器中实现。

3. 内存分配器可以添加一个方法来查找可用的内存块，而不是在进程类中实现。

4. 进程类可以将内存块的起始位置和长度作为属性，而不是在set_memory方法中设置。

下面是改进后的代码：

class Process:
    def __init__(self, pid, duration):
        self.__pid = pid
        self.__duration = duration
        self.__memory_start = None
        self.__memory_length = None

    @property
    def pid(self):
        return self.__pid

    @property
    def duration(self):
        return self.__duration

    @property
    def memory_start(self):
        return self.__memory_start

    @property
    def memory_length(self):
        return self.__memory_length

    def set_memory(self, start, length):
        self.__memory_start = start
        self.__memory_length = length

    def release_memory(self):
        self.__memory_start = None
        self.__memory_length = None


class MemoryAllocator:
    def __init__(self, memory_size):
        self.__memory_blocks = [False] * memory_size

    def find_free_memory(self, length):
        for i in range(len(self.__memory_blocks) - length + 1):
            if all(not self.__memory_blocks[j] for j in range(i, i + length)):
                return i
        return None

    def allocate_memory(self, process):
        length = process.duration
        start = self.find_free_memory(length)
        assert start is not None, 'allocation failed'
        for i in range(start, start + length):
            self.__memory_blocks[i] = True
        process.set_memory(start, length)

    def free_memory(self, process):
        start = process.memory_start
        length = process.memory_length
        for i in range(start, start + length):
            self.__memory_blocks[i] = False
        process.release_memory()

改进后的代码更加清晰和易于维护，也更加高效。

class ParkingSpot: def init(self, spot_id, status='Available'): self.spot_id = spot_id self.status = status def str(self): return f'Spot ID: {self.spot_id} Status: {self.status}' class ParkingLot: def init(self, num_spots): self.spots = [ParkingSpot(i) for i in range(1, num_spots+1)] def park_vehicle(self, spot_id): spot = self.spots[spot_id-1] if spot.status == 'Available': spot.status = 'Occupied' print(f'Spot {spot_id} is now occupied.') else: print(f'Spot {spot_id} is already occupied.') def release_spot(self, spot_id): spot = self.spots[spot_id-1] if spot.status == 'Occupied': spot.status = 'Available' print(f'Spot {spot_id} is now available.') else: print(f'Spot {spot_id} is already available.') def display_spots(self): for spot in self.spots: print(spot)具体解释代码意思

这段代码定义了一个停车场类(ParkingLot)和一个停车位类(ParkingSpot)。在停车场类中，初始化方法__init__()中接收一个参数num_spots表示停车位数量，会生成num_spots个停车位对象存储在spots列表中。类方法park_vehicle()和release_spot()可以分别用于停车和释放停车位，根据停车位状态（是否被占用）进行相应操作，并输出相应提示信息。类方法display_spots()用于展示当前停车位的状态信息。停车位类中，初始化方法__init__()中接收一个参数spot_id表示停车位编号，同样会根据编号生成一个状态(status)字符串，如果没有传入状态参数，则默认为'Available'。类方法str()可以返回当前停车位的状态信息。