Comments within SQL statements can make your application easier to read and maintain. A comment can be placed between any keywords, parameters, or punctuation marks in a statement.
For example, you can add a comment in a statement to describe the purpose of the statement in your application. Comments, except for hints, within SQL statements have no effect on the execution of the statement.
You can add a comment to a statement in one of the following ways:
Start the comment with a slash and an asterisk (/*).
Add the comment text after the slash and the asterisk. The text can span multiple lines and must end with an asterisk and a slash (*/). Spaces or line breakers are not required between the comment content and opening or closing characters.
Start the comment with two hyphens (–).
Add the comment text after the two hyphens and a space. The text cannot extend to a new line and must end with a line break.
An SQL statement can contain multiple comments in both styles. The comment text can contain any printable character in the database character set.
The following example shows a comment that starts with a slash and an asterisk (/*).
SELECT last_name, emp_id, salary + NVL(comm_pct, 0),
job_id, e.dept_id
/* Select employees whose salary is higher than that of Zhangsan.*/
FROM emp e, dept d
/* The dept table stores department names.*/
WHERE e.dept_id = d.dept_id
AND salary + NVL(comm_pct,0) > /* Subquery: */
(SELECT salary + NVL(comm_pct,0)
FROM emp
WHERE last_name = 'Zhangsan')
ORDER BY last_name, emp_id;
The following example shows a comment that starts with two hyphens (–).
SELECT last_name, -- Select the employee name,
employee_id -- employee ID,
salary + NVL(commission_pct, 0), -- total salary,
job_id, -- job title,
e.department_id -- department
FROM employees e, -- from the employees table.
departments d
WHERE e.department_id = d.department_id
AND salary + NVL(commission_pct, 0) >
(SELECT salary + NVL(commission_pct,0)
FROM employees
WHERE last_name = 'Zhangsan')
ORDER BY last_name
employee_id
;