Joins: Joins are statements that retrieve data from more than one table. Characterized in FROM clause, and relationship defined through WHERE clause. One row set is called inner, and the other is called outer.

Executes join: To execute join statement optimizer decide about access path (Chooses one or more available access paths / Execution plan), join method (nested loop, sort merge, Cartesian, and hash joins) & join order (joins two of the tables and then joins the resulting row source to the next table)

Execution plan choice:
1- Optimizer joins tables those have UNIQUE or PRIMARY KEY and then join the remaining set of tables.
2- Optimizer generates set of execution plans, according to possible join orders, join methods, and available access paths. The optimizer then estimates the cost of each plan and chooses the one with the lowest cost.