How do I create a query on more than two tables?

To create a relationship between more than two tables in SQL we can use the traditional method, or the JOIN statement.

Imagine the following tables, composed of the respective fields:

• authors
  • aut_id
  • aut_nome
• publishers
  • edi_id
  • edi_nome
• books
  • lib_id
  • lib_autore
  • lib_editore
  • lib_titolo

The field lib_autore, numerical, will have the same value as the id of the author of the homonymous table. The same holds true for the field lib_editore.

We see the classic method to write the SQL code:

 SELECT * FROM authors, publishers, books







 WHERE







 aut_id = lib_autore







 AND







 edi_id = lib_editore

We see the method that provides education JOIN:

 SELECT * FROM authors







 INNER JOIN books







 ON = autori.aut_id libri.lib_autore







 INNER JOIN publishers







 ON = libri.lib_editore editori.edi_id