Oracle9i SQL Reference Release 1 (9.0.1) Part Number A9012501 

Operators, 2 of 6
Operators manipulate individual data items called operands or arguments. Operators are represented by special characters or by keywords. For example, the multiplication operator is represented by an asterisk (*).
Note:
If you have installed Oracle Text, you can use the 
The two general classes of operators are:
Other operators with special formats accept more than two operands. If an operator is given a null operand, the result is always null. The only operator that does not follow this rule is concatenation ().
Precedence is the order in which Oracle evaluates different operators in the same expression. When evaluating an expression containing multiple operators, Oracle evaluates operators with higher precedence before evaluating those with lower precedence. Oracle evaluates operators with equal precedence from left to right within an expression.
Table 31 lists the levels of precedence among SQL operators from high to low. Operators listed on the same line have the same precedence.
Operator  Operation 


identity, negation 

multiplication, division 

addition, subtraction, concatenation 

In the following expression, multiplication has a higher precedence than addition, so Oracle first multiplies 2 by 3 and then adds the result to 1.
1+2*3
You can use parentheses in an expression to override operator precedence. Oracle evaluates expressions inside parentheses before evaluating those outside.
SQL also supports set operators (UNION
, UNION
ALL
, INTERSECT
, and MINUS
), which combine sets of rows returned by queries, rather than individual data items. All set operators have equal precedence.

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