In a join operation involving two tables,
scans the tables in a
particular order.
accesses rows in one table first, and this table is now called the outer
table.
Then, for each qualifying row in the outer table,
looks for matching rows
in the second table, which is called the inner table.
accesses the outer
table once, and the inner table probably many times (depending on how many rows
in the outer table qualify).
This leads to a few general rules of thumb about join order: