Rules of
Precedence
The rules
of precedence determine the order in which expressions are evaluated and
calculated. The table above lists the default order of precedence. However, you
can override the default order by using parentheses around the expressions that
you want to calculate first.
1- Precedence of the AND Operator: Example
In this
example, there are two conditions:
•
The
first condition is that the job ID is AD_PRES and the salary is greater
than $15,000.
•
The
second condition is that the job ID is SA_REP.
Therefore,
the SELECT statement reads as follows:
“Select the
row if an employee is a president and earns more than $15,000, or
if the employee is a sales representative.”
2- Using Parentheses: Example
In this
example, there are two conditions:
•
The
first condition is that the job ID is AD_PRES or SA_REP.
•
The
second condition is that the salary is greater than $15,000.
Therefore,
the SELECT statement reads as follows:
“Select the
row if an employee is a president or a sales representative, and
if the employee earns more than $15,000.”
SELECT
last_name, job_id, salary FROM
employees WHERE job_id = 'SA_REP'
OR job_id = AD_PRES' AND salary > 15000;
SELECT
last_name, job_id, salary FROM
employees WHERE (job_id =
'SA_REP' OR job_id = 'AD_PRES') AND salary > 15000;
