SQL Comments

In SQL, comments are descriptions in the code that help users better understand the intent and functionality of the SQL command. For example,

/* This is a multi-line
comment in SQL. */

-- This is a single-line comment in SQL.

They are completely ignored by database management systems.

Types of Comments

There are mainly two types of comments in SQL. They are:

• Single-line Comments
• Multi-line Comments

Single-line Comments

In SQL, we use the double dash, --, to write a single-line comment. The comment starts from -- and ends with the end of line. For example,

-- fetch all records from the Students table
SELECT *
FROM Students;

Here, the comment is

-- fetch all records from the Students table

Database systems completely ignore this line during code sql-execution.

Comments With Statements

It's also possible to include comments in the same line as an sql-executable SQL statement. For example,

SELECT * -- select all records
FROM Students; -- from the Students table

Here, the comments are

• -- select all records
• -- from the Students table

Multi-line Comments

In SQL, a multi-line comment starts with /* and ends with */. For example,

/* selecting all records
from the
Students table */
SELECT *
FROM Students;

Here, anything between /* and */ is a comment and is ignored by database management systems.

Comments Within Statements

Similar to single-line comments, it's also possible to include multi-line comments in the same line as an sql-executable SQL statement. For example,

SELECT *
FROM /* table name here */ Students;

Here, anything between /* and */ is a comment and is ignored by database management systems.

Using Comments to Debug Code

Suppose, we want to skip certain SQL statements from sql-execution. In such cases, instead of removing the statements, we can simply comment it out.

This helps us to test our SQL code without removing them completely. For example,

/* SELECT *
FROM Customers; */
-- the above statement is ignored by DBMS

SELECT *
FROM Students;

Here, SQL will only fetch records from the Students table while ignoring the command to fetch records from the Customers table.