Oracle操作基础.ppt

* Operator Precedence If an arithmetic expression contains more than one operator, multiplication and division are evaluated first. If operators within an expression are of same priority, then evaluation is done from left to right. You can use parentheses to force the expression within parentheses to be evaluated first. * Operator Precedence (continued) The example on the slide displays the last name, salary, and annual compensation of employees. It calculates the annual compensation as 12 multiplied by the monthly salary, plus a one-time bonus of $100. Notice that multiplication is performed before addition. Note: Use parentheses to reinforce the standard order of precedence and to improve clarity. For example, the expression on the slide can be written as (12*salary)+100 with no change in the result. Instructor Note Demo: 1_prec1.sql, 1_prec2.sql Purpose: To illustrate viewing a query containing no parentheses and executing a query with parentheses to override rules of precedence. * Character Strings and Dates Character strings and dates in the WHERE clause must be enclosed in single quotation marks (). Number constants, however, should not be enclosed in single quotation marks. All character searches are case sensitive. In the following example, no rows are returned because the EMPLOYEES table stores all the last names in mixed case: SELECT last_name, job_id, department_id FROM employees WHERE last_name = WHALEN; Oracle databases store dates in an internal numeric format, representing the century, year, month, day, hours, minutes, and seconds. The default date display is DD-MON-RR. Note: Changing the default date format is covered in a subsequent lesson. Instructor Note Some students may ask how to override the case sensitivity. Later in the course, we cover the use of single-row functions such as UPPER and LOWER to override the case sensitivity. * Using the Comparison Conditions In the example, the SELECT statement retrieves the last nam