SQL | Comments

Comments are used to explain sections of SQL statements or to prevent SQL statements from being executed. As is any programming language comments matter a lot in SQL also. In this set, we will learn about writing comments in any SQL snippet.

Comments can be written in the following three formats:

1. Single-line comments
2. Multi-line comments
3. In-line comments

Single Line Comments

Comments starting and ending in a single line are considered single-line comments. A line starting with ‘–‘ is a comment and will not be executed. 

Syntax:

— single line comment

— another comment

SELECT * FROM Customers; 

Let’s see an example in which we have one table name Customers in which we are fetching all the data records.

Query:

SELECT * FROM customers; 
-- This is a comment that explains 
   the purpose of the query.

Multi-Line Comments

Comments starting in one line and ending in different lines are considered as multi-line comments. 

A line starting with ‘/*’ is considered as starting point of the comment and is terminated when ‘*/’ is encountered. 

Syntax:

/* multi line comment

another comment */

SELECT * FROM Customers; 

Let’s consider that we want to fetch order_date with the year 2022.

Query:

/*
This is a multi-line comment that explains 
the purpose of the query below.
The query selects all the orders from the orders 
table that were placed in the year 2022.
*/
SELECT * FROM orders WHERE YEAR(order_date) = 2022;

In-Line Comments

In-line comments are an extension of multi-line comments, comments can be stated in between the statements and are enclosed in between ‘/*’ and ‘*/’. 

Syntax:

SELECT * FROM /* Customers; */ 

Let’s Suppose we have on a table with name orders and we are fetching customer_name.

Query:

SELECT customer_name, 
/* This column contains the name of 
the customer / order_date / 
This column contains the date the
order was placed */ FROM orders;

More Examples:

There are a few more examples of Multiline comments and inline comments.

Multi line comment ->
/* SELECT * FROM Students;
SELECT * FROM STUDENT_DETAILS;
SELECT * FROM Orders; */
SELECT * FROM Articles; 

In line comment ->
SELECT * FROM Students;
SELECT * FROM /* STUDENT_DETAILS;
SELECT * FROM Orders;
SELECT * FROM */ Articles; 

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