Comments on SQL statements

The comment in an SQL statement improves the readability and maintainability of the application. Comments can be inserted between any keywords, parameters, or punctuation in the statement.

For example, you can use comments in your statements to describe the purpose of the statements in the application. Other than hints, comments in SQL statements do not affect the execution of the statements.

You can add comments in the following two ways:

• Comments that start with a forward slash and an asterisk (/*).

  A comment consists of the slash ( /) and asterisk ( *), and text that follows. This text can span multiple lines and is terminated by an asterisk and slash ( */). The comment does not have to be separated from the text by spaces or line breaks.

• Single-line comments start with two hyphens ( -- ).

  Text following symbols and comments with a space between, not extended to new lines, and ending with a line break.

A single SQL statement can contain multiple comments from two different styles. The text within a comment can contain any printable character in the database character set.

The following example shows comments that start with the slash and asterisk (*) symbols.

SELECT last_name, emp_id, salary + NVL(comm_pct, 0),
            job_id, e.dept_id
  /* Selects the employees whose salary is higher than that of Zhangsan.*/
  FROM emp e, dept d
  /*DATA IN DEPT TABLE IS DEPARTMENT NAME.*/
  WHERE e.dept_id = d.dept_id
    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's name.
       employee_id                                   -- employee id
       salary + NVL(commission_pct, 0),             -- total compensation
       job_id,                                      -- Job name
       e.department_id                              -- Department
  FROM employees e,                                 -- Select from all employees.
       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
;