SQL Query to Count() multiple tables

Question

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

What is the best way to do this?

Answer 1

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

Using inline views:

   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

Answer 2

you could use the COUNT(distinct x.id) synthax:

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