Why do multiple-table joins produce duplicate rows?

Question

Let\'s say I have three tables A, B, and C. Each has two columns: a primary key and some other piece of data. They each have the same number of rows. If I JOIN

Answer 1

Ok in this example you are getting duplicates because you are joining both D and S onto M. I assume you should be joining D.id onto S.id like below:

SELECT *
FROM M
INNER JOIN S
    on M.Id = S.Id
INNER JOIN D
    ON S.Id = D.Id
INNER JOIN H
    ON D.Id = H.Id