假设我有两个现有表，“ dogs”和“ cats”：
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
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
