Comparison Operators

Comparison operators are used with character, numeric, or date data and can be used in the WHERE or HAVING clause of a query. Comparison operators evaluate to a Boolean data type; they return TRUE or FALSE based on the outcome of the tested condition.

For example, to calculate a bonus for those employees who have been hired on or before March 15, 1998, a computation of whether the hire_date for an employee is less than or equal to March 15, 1998 provides the list of employees who should receive bonuses.

Valid comparison operators are:

• > (greater than).

• < (less than).

• = (equals).

• <= (less than or equal to).

• >= (greater than or equal to).

• != (not equal to).

• <> (not equal to).

• !< (not less than).

• !> (not greater than).

Comparison operators can also be used in program logic to check for a condition. For example, if the country column is UK rather than Spain, different shipping rates may apply. In this case, a combination of a comparison operator, an expression (the column name), a literal (‘UK’) and a control-of-flow programming keyword (IF) are used together to achieve this purpose.

Anyone with access to the actual data (for queries) can use comparison operators in additional queries. For those data-modification statements, it is recommended that you use comparison operators only if you know you have the appropriate permissions and that data will be changed by only a limited group of people (to maintain data integrity).

Queries also use string comparisons to compare the value in a local variable, cursor, or column with a constant. For example, all customer rows should be printed if the country is the UK. The table shows string comparison examples between Unicode and non-Unicode data; ST1 is char and ST2 is nchar.