SQL查询到Count（）多个表

我有一个表，它与其他表有几个一对多的关系。假设主表是一个人，其他表代表宠物、汽车和儿童。我想要一个查询，返回个人的详细信息，宠物，汽车和孩子的数量，例如

Person.Name Count(cars) Count(children) Count(pets) John Smith 3 2 4 Bob Brown 1 3 0

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最好的方法是什么？

注意，这取决于您对RDBMS的喜好，以及它是否支持以下嵌套选择：

SELECT p.name AS name
   , (SELECT COUNT(*) FROM pets e WHERE e.owner_id = p.id) AS pet_count
   , (SELECT COUNT(*) FROM cars c WHERE c.owner_id = p.id) AS world_pollution_increment_device_count
   , (SELECT COUNT(*) FROM child h WHERE h.parent_id = p.id) AS world_population_increment
FROM person p
ORDER BY p.name

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IIRC，这至少适用于PostgreSQL和MSSQL。未测试，因此您的里程数可能会有所不同。

使用子选择不是很好的做法，但可能在这里它会很好

select p.name, (select count(0) from cars c where c.idperson = p.idperson), (select count(0) from children ch where ch.idperson = p.idperson), (select count(0) from pets pt where pt.idperson = p.idperson) from person p

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可以使用三个外部联接来执行此操作：

SELECT
    Person.Name,
    sum(case when cars.id is not null then 1 else 0 end) car_count,
    sum(case when children.id is not null then 1 else 0 end) child_count,
    sum(case when pets.id is not null then 1 else 0 end) pet_count
FROM
    Person
LEFT OUTER JOIN
    cars on
    Person.id = cars.person_id
LEFT OUTER JOIN
    children on
    Person.id = children.person_id
LEFT OUTER JOIN
    pets on
    Person.id = pets.person_id
GROUP BY
    Person.Name

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我相信Oracle现在支持case when语法，但如果不支持，您可以使用解码。

我可能会这样做：

SELECT Name, PersonCars.num, PersonChildren.num, PersonPets.num
FROM Person p
LEFT JOIN (
   SELECT PersonID, COUNT(*) as num
   FROM Person INNER JOIN Cars ON Cars.PersonID = Person.PersonID
   GROUP BY Person.PersonID
) PersonCars ON PersonCars.PersonID = p.PersonID
LEFT JOIN (
   SELECT PersonID, COUNT(*) as num
   FROM Person INNER JOIN Children ON Children.PersonID = Person.PersonID
   GROUP BY Person.PersonID
) PersonChildren ON PersonChildren.PersonID = p.PersonID
LEFT JOIN (
   SELECT PersonID, COUNT(*) as num
   FROM Person INNER JOIN Pets ON Pets.PersonID = Person.PersonID
   GROUP BY Person.PersonID
) PersonPets ON PersonPets.PersonID = p.PersonID

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您需要在查询中包含多个count语句。在我的头顶上

SELECT  p.Name, COUNT(DISTINCT t.Cars), COUNT(DISTINCT o.Children), Count(DISTINCT p.Pets)
FROM Person p
INNER JOIN Transport t ON p.ID = t.PersonID
LEFT JOIN Offspring o ON p.ID = o.PersonID
LEFT JOIN Pets p ON p.ID = o.OwnerID
GROUP BY p.Name
ORDER BY p.Name

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子查询分解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

您可以使用COUNTdistinct x.id synthax：

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这是最简单的解决方案，但可能会由于相关子查询而导致性能低下。如果你的数据库很小，这可能无关紧要。有趣的是，你给出了你个人风格的技术答案@埃里克：你说得对，不过，争议裁决委员会可能会重写这句话。因此，最终它可能会成为您提出的建议。睡眠剥夺可能会导致自我克制的丧失：-Oracle自9i以来一直支持案例陈述。Rexem，感谢您的澄清！我写了一些令人讨厌的解码语句，我希望能够使用case语句！道格。你也可以只使用countcars.id等。它不会计算空值。投票支持这个，因为它非常简单。

SELECT person.name, 
       COUNT(DISTINCT car.id) cars, 
       COUNT(DISTINCT child.id) children, 
       COUNT(DISTINCT pet.id) pets
  FROM person
  LEFT JOIN car ON (person.id = car.person_id)
  LEFT JOIN child ON (person.id = child.person_id)
  LEFT JOIN pet ON (person.id = pet.person_id)
 GROUP BY person.name