Question

I have been trying to understand joins for a while now, I’ve noticed quite a few examples showing how to retrieve a value based on a two-table query using a left / inner join. I will try to draw out my example for you all, hopefully it will aid in the explanation of my question.

Table 1:
project_id  |  project_name
    1          super-project

Table 2:
project_id  |  course_id       ---> project_id and course_id are foreign keys here.
    1              3

Table 3:
course_id  |  course_name      ---> course_id is a primary key here
    3      |  Writing 101

My Goal:

I would like to be able to return “Writing 101” by using the project_id. So, if I know the project_id is 1, I would query the db to find that the course_id associated with the current project is 3. I would then use the course_id of 3 to query table 3 and find that the related course_name is writing 101.

Is there a way to do this in a single query using join of some sort?

Answer 1

select course_name from Table3
inner join Table2
on Table2.course_id = Table3.course_id
where Table2.project_id = 1

Notice there is only 1 join needed!

Also note that if the project_id = 1 is present in more than one course all of those courses will appear.

Can you explain why only one join is needed as opposed to the answer below which requires two joins?

The other answer does not require two joins. As the project_id is present in the “middle” table there is no need to look for it in Table1. Now, if you require the name of the project (which is not the case because you didn’t mention that in the question) then you would have to join the Table1