Joins: Commutativity

When an operation is commutative, the ordering of elements does not matter.

Addition and multiplication are both commutative. You can change the order of the elements. The result is always the same.

Addition

\[1 + 2 + 3 = 6\\ 3 + 2 + 1 = 6\\\]

\[1 \times 2 \times 3 = 6\\ 3 \times 2 \times 1 = 6\\\]

A INNER JOIN B = B INNER JOIN A

Left/Right joins are not commutative.

That is

A LEFT JOIN B <> B LEFT JOIN A

But:

A LEFT JOIN B = B RIGHT JOIN A

http://blog.ylett.com/2011/09/non-associativity-of-sql-table-joins.html http://www.sql-tutorial.ru/en/book_explicit_join_operations/page4.html