JOIN's are making my brain hurt :) oh GROUPBY?

[Demonic]

Alright I understand how to use a basic "JOIN" clause, but whats up with all this:

 

LEFT JOIN, RIGHT JOIN, INNER JOIN, OUTTER JOIN

 

I'm at the basic join lol whats needed for the other stuff?  While we're on the subject, groupby, I was reading on w3schools about groupby, whats another use for groupby besides averaging results, I only understand GroupBy to be used to average columns and what not.

[fenway]

LEFT and RIGHT are OUTER joins.

JOIN is an INNER join (it's implied).

 

Usually, I would write INNER JOIN vs. LEFT JOIN to make the distinction.

 

LEFT vs. INNER -- normally, with an INNER join, if there are non-matching rows based on the join condition, no rows will be returned.  With a LEFT join, rows from the first table with non-matching rows will yield NULLed out column for the fields in the second table -- making it possible to find non-matches (like movie reviews without votes, etc.).  RIGHT is the opposite, but the convention is to use LEFT JOIN.