在单个 mysql 查询中查找计数和数据

Question

我有两张桌子。城市，地区。

Cities:  
    id | name  
Localities:  
    id | city_id | name

我可以通过一次查询来获取城市的地点列表（每个城市只有 10 行），以及该城市的地点数吗？

我知道我可以在 2 个不同的查询中做到这一点。

请建议是否可以使用 Django 查询集来完成。

型号：

class City(models.Model):
    name = models.CharField(max_length=200)    

class Locality(models.Model):
    name = models.CharField(max_length=200)
    city = models.ForeignKey(City,related_name='LocalityCity')

Answer 1

select * from
( select c.id as city_id, count(l.id) as "localities count per city"
  from cities as c
  inner join localities as l on l.city_id = c.id ) as localities_count_per_city
inner join
( select c.id as city_id, c.name, l.name
  from cities as c
  inner join localities as l on l.city_id = c.id
  limit 10) as localities_per_city ) as localities_per_city
on localities_count_per_city.city_id = localities_per_city.city_id

这应该将每个城市计数的各个地方与城市行中的每个地方连接起来。

Answer 2

您应该提到的是模型而不是 SQL 表模式。以下是查询它们的方法：

cities = City.objects.annotate(num_cities=Count("locality"))
for city in cities:
    print(city.locality_set, city_num_cities)

Answer 3

我可以进行一次查询以获取 城市（每个城市只有 10 行），也是地方的计数 那个城市？

假设您的模型是：

class Locality(models.Model):
    name = models.CharField(max_length=200)    

class City(models.Model):
    name = models.CharField(max_length=200)
    locality = models.ManyToMany(Locality)

你可以得到这样的结果：

all_cities = City.object.all()
for city in all_cities:
   print('City: {} Total Localities: {}'.format(city.name, city.locality_set.count()))
   for loc in city.locality_set.all():
       print('\t{}'.format(loc.name))

在上面的 sn-p 中，有 两个 查询正在执行 - 但 django 查询集被缓存，因此您可以放心每次都执行正确的查询。

如果你想一次完成，那么你需要使用聚合：

from django.db.models import Count

all_cities = City.objects.all().annotate(localities=Count('locality'))
for city in all_cities:
   print('City: {} Total Localities: {}'.format(city, city.localities))