SQL查询到Count()多个表 [英] SQL Query to Count() multiple tables

问题描述

我有一个表，该表与其他表具有多个一对多的关系.假设主表是一个人，其他表则代表宠物，汽车和儿童.我想要一个查询，该查询返回人员的详细信息，例如他们拥有的宠物，汽车和儿童的数量.

I have a table which has several one to many relationships with other tables. Let's say the main table is a person, and the other tables represent pets, cars and children. I would like a query that returns details of the person,the number of pets, cars and children they have e.g.

Person.Name   Count(cars) Count(children) Count(pets)

John Smith    3           2               4
Bob Brown     1           3               0

做到这一点的最佳方法是什么?

What is the best way to do this?

推荐答案

子查询分解(9i +):

Subquery Factoring (9i+):

WITH count_cars AS (
    SELECT t.person_id
           COUNT(*) num_cars
      FROM CARS c
  GROUP BY t.person_id),
     count_children AS (
    SELECT t.person_id
           COUNT(*) num_children
      FROM CHILDREN c
  GROUP BY t.person_id),
     count_pets AS (
    SELECT p.person_id
           COUNT(*) num_pets
      FROM PETS p
  GROUP BY p.person_id)
   SELECT t.name,
          NVL(cars.num_cars, 0) 'Count(cars)',
          NVL(children.num_children, 0) 'Count(children)',
          NVL(pets.num_pets, 0) 'Count(pets)'
     FROM PERSONS t
LEFT JOIN count_cars cars ON cars.person_id = t.person_id
LEFT JOIN count_children children ON children.person_id = t.person_id
LEFT JOIN count_pets pets ON pets.person_id = t.person_id

使用内联视图:

   SELECT t.name,
          NVL(cars.num_cars, 0) 'Count(cars)',
          NVL(children.num_children, 0) 'Count(children)',
          NVL(pets.num_pets, 0) 'Count(pets)'
     FROM PERSONS t
LEFT JOIN (SELECT t.person_id
                  COUNT(*) num_cars
             FROM CARS c
         GROUP BY t.person_id) cars ON cars.person_id = t.person_id
LEFT JOIN (SELECT t.person_id
                  COUNT(*) num_children
             FROM CHILDREN c
         GROUP BY t.person_id) children ON children.person_id = t.person_id
LEFT JOIN (SELECT p.person_id
                  COUNT(*) num_pets
             FROM PETS p
         GROUP BY p.person_id) pets ON pets.person_id = t.person_id

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