sql - DB: advantages of relations -

I always think that relationships between tables are necessary for cross-table operations, such as join. But I came to know that I could internally engage in two tables which are not exactly linked (no foreign key).

Then, my question:

1. What are some differences in joining in linked and non-linked tables (like speed) Are you
2. What are the advantages / disadvantages of using relationships between tables?

   Thank you in advance. The primary advantage is that foreign key constraints ensure the relational integrity of the data. That is, it will give you such an example.

   If you have your own FK