Another reminder note.

The RDBMS query engine relies on these laws to build efficient queries:

Commutation:

Numbers may travel (commute)

or in relational algebra

Association:

Numbers may freely associate

or in relational algebra

Distribution:

Operators can be distributed

or, in relational algebra

A more general statement of these laws:

Commutative law:

Associative law:

Distributive law:

Date has an excellent discussion of all this on pp 124 – 127 of SQL and Relational Theory: How to Write Accurate SQL Code

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