SQL可以跨多个表计算聚合函数吗？

Question

假设我有两个现有的桌子,"狗"和"猫":

 dog_name | owner
 ---------+------
 Sparky   | Bob
 Rover    | Bob
 Snoopy   | Chuck
 Odie     | Jon

 cat_name | owner
 ---------+------
 Garfield | Jon
 Muffy    | Sam
 Stupid   | Bob

如何使用此输出编写查询？

 owner | num_dogs | num_cats
 ------+----------+---------
 Bob   |     2    |    1
 Chuck |     1    |    0
 Sam   |     0    |    1
 Jon   |     1    |    1

Answer 1

select owner, sum(num_dogs), sum(num_cats) from
  (select owner, 1 as num_dogs, 0 as num_cats from dogs
   union
   select owner, 0 as num_dogs, 1 as num_cats from cats)
group by owner

Answer 2

我更喜欢这个:

select owner
     , count(dog_name) dogs
     , count(cat_name) cats
  from cats FULL OUTER JOIN dogs ON (cats.owner = dogs.owner)