LEFTJOIN, Rightjoin in MySQL
left (outer) join: The left table is the main, including all the rows of the left table, and the right table satisfies the record data list
right (outer) join: the right table is the main, including all the rows of the right table, and the left table satisfies the record data list
cross join: Cartesian product (all possible combinations of rows).
inner join: The cross combination that satisfies the join condition.
full outer join: superset of all rows of left outer and right outer.