Summary -

In this topic, we described about the below sections -

What is SQL Comment?

SQL Comments are used in SQL Statements to understand others easily what that statement purpose, how it works on the database. The one who writes the query understood easily, because he had an idea what data he wants to fetch from database, but others face a difficulty in understanding. Due to some complex queries like nested queries, joins it became complex to understand the query. So, we can explain the query in comments with little information for understanding easily.

In other side, comment also used to skip the execution of some part of the statement.

SQL Comment types

SQL Comments will not affect the SQL query execution. SQL allows two types comments. They are as follows -

  • Single Line Comments
  • Multi-line Comments

Single Line Comments

To write single line comments, Double hyphens (--) are used the beginning of the comment. It contains single row of information. The text should not go to the newline. The text after the double hyphens(--) considered s comment and won't get executed. The single line comment always start with '--'.


Example-

Consider an employee table with 9 employee's information like eid, ename, destination, etc,.

employee_details -

emp_id emp_name designation manager_id date_of_hire salary dept_id
001 Employee1 Director 2019-11-07 45000.00 1000
002 Employee2 Director 2019-11-07 40000.00 2000
003 Employee3 Manager Employee1 2019-11-07 25000.00 1000
004 Employee4 Manager Employee2 2019-11-07 24000.00 2000
005 Employee5 Analyst Employee3 2019-10-08 20000.00 1000
006 Employee6 Analyst Employee3 2019-10-08 20000.00 1000
007 Employee7 Clerk Employee3 2019-10-08 15000.00 1000
008 Employee8 Salesman Employee4 2019-10-08 15000.00 2000
009 Employee9 Salesman Employee4 2019-10-08 15000.00 2000

Let us assume, we are trying to retrieve employee name, designation, and salary details from employee table. The query for the same as follows -

-- Single line comment
-- Query to fetch employee name, designation, and salary from table
    SELECT ename, designation, salary FROM employee_details; 

-- The above query can also written like below -
SELECT ename,  -- Employee Name
       designation, -- Employee designation
       salary   -- Employee Salary
    FROM employee_details;  

Output-

emp_name designation salary
Employee1 Director 45000.00
Employee2 Director 40000.00
Employee3 Manager 25000.00
Employee4 Manager 24000.00
Employee5 Analyst 20000.00
Employee6 Analyst 20000.00
Employee7 Clerk 15000.00
Employee8 Salesman 15000.00
Employee9 Salesman 15000.00

Explaining Example -

  • In the above first query, comment line gives the information about what the statement used to do.
  • In the second query, the comment specifies the column characteristics.

Multi-line Comments

Multi-row comments are used either to write multiple lines of information about statement or to ignore the multiple statements. The comments are beginning with slash followed by Asterix (/*) and end with Asterix followed by slash (*/).


Example -

Consider an employee table with 9 employee's information like eid, ename, destination, etc,.

employee_details -

emp_id emp_name designation manager_id date_of_hire salary dept_id
001 Employee1 Director 2019-11-07 45000.00 1000
002 Employee2 Director 2019-11-07 40000.00 2000
003 Employee3 Manager Employee1 2019-11-07 25000.00 1000
004 Employee4 Manager Employee2 2019-11-07 25000.00 2000
005 Employee5 Analyst Employee3 2019-10-08 20000.00 1000
006 Employee6 Analyst Employee3 2019-10-08 20000.00 1000
007 Employee7 Clerk Employee3 2019-10-08 15000.00 1000
008 Employee8 Salesman Employee4 2019-10-08 15000.00 2000
009 Employee9 Salesman Employee4 2019-10-08 15000.00 2000

Let us assume, we are trying to retrieve employee name, designation, and salary details from employee table. The query for the same as follows -

/* Query to fetch employee name, designation, 
        Salary details from employee_details table */ 
    SELECT ename, designation, salary FROM employee_details; 

output-

emp_name designation salary
Employee1 Director 45000.00
Employee2 Director 40000.00
Employee3 Manager 25000.00
Employee4 Manager 24000.00
Employee5 Analyst 20000.00
Employee6 Analyst 20000.00
Employee7 Clerk 15000.00
Employee8 Salesman 15000.00
Employee9 Salesman 15000.00

Explaining Example -

  • In the above example, multi-row comment line gives the information about what the statement used to do.