When you do SQL queries do you find yourself always, or almost always, using left joins?  If you've answered yes to this question, I would suggest to continue to read and understand some downfalls that you might be getting yourself into.

Let's start by looking at a standard query that has a left join:

SELECT * FROM users u LEFT JOIN countries c ON c.id = u.country_id WHERE u.id = 5

The following query will return all fields from the users table and all fields from the countries table.  Now you're thinking, "Yeah, looks good, so what's wrong with this?".  Well, I'm glad you asked.