find duplicate records in related tables

Question

Consider the following data structure:

parking garages
    |
    |_ garage 1
        |__ red car
        |__ blue car
    |_ garage 2
        |__ yellow car
        |__ orange car
    |_ garage 3
        |__ red car
        |__ red car
        |__ yellow car

"parking garages" is a table, and each entry is a foreign key to a "garageCars" table containing one record for each car in the garage.

What I would like to do is write a query that would return "garage 3" because it has 2 red cars

I've tried using the HAVING clause with a count, but this returns all the rows that have more than 1 car. I need something more along the lines of "WHERE count(car) > 1 and car.color = car.color".

Answer 1

Assuming you only want the garage name and not the color that's duped because you want "... query that would return "garage 3" because it has 2 red cars"

Using a Derived Table

SELECT g.garagename 
FROM   garage g 
       INNER JOIN (SELECT garage_id 
                   FROM   cars 
                   GROUP  BY garage_id, 
                             carcolor 
                   HAVING COUNT(garage_id) > 1) c 
         ON g.garage_id = c.garage_id 

Using IN

SELECT g.garagename 
FROM   garage g 
WHERE  g.garage_id IN (SELECT garage_id 
                       FROM   cars 
                       GROUP  BY garage_id, 
                                 carcolor 
                       HAVING COUNT(garage_id) > 1) 

You could also do it with EXISTS, CROSS APPLY or with a CTE