12.14. Summary
The data manipulation aspect of the relational model includes relational algebra and relational assignment (:=). Apart from the use of logical and comparison operators in expressing conditions, the algebra includes eight basic table operations. Comparison operators have precedence over logical operators, which have precedence over table operators. Table 12.3 listed the six priority levels of the operators: =, <>, <, >, < =, >=; not; and; or; selection (where ...) and projection ([ ]); and finally union ∪, intersection ∩, difference - (termed except in SQL), Cartesian product ×, natural join ⋈ and division ÷. Parentheses may be used to override this order.
Operators on the same precedence level are left associative (evaluated in ...
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