Though not emphasized in the SQL, operators serve their important role by enabling you to manipulate output and to specify selection criteria and search conditions.
Operators can be unary or binary, depending on whether they are applied to one or two operands.
The argument of what could be considered an operator goes on for at least as long as the existence of RDBMS — some databases insist on keywords such as LIKE, IN, EXISTS, BETWEEN and so on to be classified as operators, and some list them as predicates. This is a rather academic discussion, as classification does not affect the way you use either operators or predicates in SQL programming.
Operators are generally uniform across all database vendors with the notable exception of bitwise operators in Microsoft SQL Server and nonstandard comparison operators.
The precedence of operators is an established order in which RDBMS evaluates expressions that contain more than one operator; it is very important to take into consideration the precedence order. Using parentheses, one can specify custom precedence in an expression (as opposed to default precedence order).