Understand the joins with more than one column?

Hi all

I want to know if I understand.

When you join two tables, and the join condition has four columns (two for each table) as follows:

where d.deptno = e.deptno

and d.name = e.name

I think it means the engine must choose "the selection list" when he sees a number of d.deptno (for example 10) equal number in e.deptno, and

the d.name from the same folder (10) must be equal to the value of e.name.

I understand?

Another example, I want to understand, when we say:

where d.deptno = e.deptno

and d.deptno = s.deptno

This means that the number (10) should for example equal to one (10) in the table (e), and the same 10 must be equal to a value in the table (s)?

Am I wrong?

Thank you

Hello

Yes, what you say is right; but don't take my word for it.  Try it yourself.   Create a couple of small tables.  Insert some data that matches (and will appear in the results of joints) and others that does not (and should not be in the results) and see if the results are what you expected.

Tags: Database

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