Can SQL calculate aggregate functions across multiple tables?

Question

Let\'s say I have two existing tables, \"dogs\" and \"cats\":

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


        

Answer 1

I started with Cade Roux's excellent answer, but changed the WITH...AS () to use a table variable, as I am ended up using the results from a similar query for further aggregate functions.

-- Table variable declaration
DECLARE @RainingCatsDogs TABLE
(
    Owner nvarchar(255),
    num_cats int,
    num_dogs int
)

-- Populate the table variable with data from the union of the two SELECT statements
INSERT INTO @RainingCatsDogs

-- Get the count of doggies
SELECT 
    owner, COUNT(dog_name) AS num_dogs, 0 AS num_cats
FROM 
    dogs
GROUP BY 
    owner

-- join the results from the two SELECT statements
UNION

-- Get the count of kittehs
SELECT 
    owner, 0 AS num_dogs, COUNT(cat_name) as num_cats
FROM 
    cats
GROUP BY 
    owner

-- From the table variable, you can calculate the summed results
SELECT
    owner,
    SUM(num_dogs),
    SUM(num_cats)
FROM 
    @RainingCatsDogs