How to PROPERLY add comments in SQL [Multiple Ways]

Objective To use the comment in SQL

Comments are used to prevent line of a statement from the execution, Comments can make your application easier for you to read and maintain, Comments are used to explain sections of SQL statements,

A comment can appear between any keywords, parameters, or punctuation marks in a statement. You can include a comment in a statement in three ways:

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

 

Sample Employee Table

We will use the following Employee Table through out this article:

 

How to add Single line comment in SQL

• The comments which start and end in a single line are considered as single-line comments
• Single line comments begin with -- (two hyphens) Proceed with the text of the comment, this text cannot extend to a new line, End the comment with a line break

 

Syntax of SQL single-line comment

-- single line comment
-- another comment
Lines of code to be executed 

 

Examples of SQL single-line comment

Example 1: Write SQL query with the comment to specify what will be the result of the query and which line of the statement will be executed

--Below query is used to display all employee data
--Resulting record set is having column heading same as the column name 
SELECT        Emp_id, Emp_name, street, city, Emp_contact, Salary, Dept_id
FROM            tblemp

• In the above example, two single-line comments is given using ‘—‘ two hyphens symbol
• Below the comment, we have specified SQL select statement query which will be executed and return all employee details

OUTPUT:

 

How to add Multi line comment in SQL

• The Comments which starts in one line and ended in different line are considered multi-line comments.
• SQL multiline comments is starting with ‘/*’ as the starting point of comment and are terminated when ‘*/’ is encountered

 

Syntax of SQL Multi-line comment

/* multi line comment
another comment */
Lines of code to be executed;

 

Example of SQL Multi line comment

Example 2: Write SQL query with the comment to specify what will be the result of the query and which line of the statement will be executed

/* Select all the columns
of all the records
in the Employee table */
SELECT  Emp_id, Emp_name, street, city, Emp_contact, Salary, Dept_id
FROM   tblemp

• In the above query, SQL multi-line comment is given on three lines using ‘/* */’
• Below the comment lines, we specified SQL select query specified which executed and returns all employee records with all column values

OUTPUT:

 

How to add inline comment in SQL

• Inline, comments are an extension of SQL multi-line comments
• SQL Inline comments can be stated in between the statements and are enclosed in between ‘/*’ and ‘*/’
• To ignore just a part of a statement, we can also use the /* */ SQL multi-line comment which is called as Inline comment

 

Syntax of SQL Inline comment

Lines of code to be /* comment in the code */ executed;

 

Example of SQL Inline comment

Example 3: Write SQL query with a comment within the query to not to execute some query code part and merge with uncommenting part

SELECT * FROM tblemp;  
SELECT * FROM /* tblcompany;  
SELECT * FROM tblorders;  
SELECT * FROM */ tbldept;  

• In the above list of queries, SQL inline comment is used to make comments on some part of the query using /* and */
• The query part which is in-between /*… */ will not be executed so the only first query will be executed and returns all records of tblemp

OUTPUT:

 

How to add Nested SQL Comment

We can have nested comments in SQL Server.  if you specify the block open (/*) in existing comments, SQL Server treats it as a nested comment. Therefore, the nested comment should have a corresponding block closed(/*) comment mark. In case you do not specify a nested comment block close mark, you get an error message

 

Example of Nested SQL Comment

DECLARE @Text AS VARCHAR(20);  
GO  
/*   --Outer Block opened 
SELECT @comment = '/*';  --Nested Block opened 
*/ --Nested Block closed 
*/  --Outer Block Closed
SELECT @@VERSION;  
GO

• In the above query, we have used SQL Nested comment by specifying SQL multi-line comment within the comment
• We started multi-line comment with outer opening /* and before closing outer comment we have started inner nested comment with /* and closed it in next line with */ symbol

OUTPUT:

 

SQL Comment Indicators

• SQL Comment Indicator is indicated according to the given examples
• It includes the double hyphen ( — ), braces ( { } ), and C-style ( /* . . . */ ) comment delimiters. It also includes the comments after the statement

 

SQL Comment Indicators Examples

Consider employee table to perform practical example

SELECT * FROM tblemp; -- Selects all rows and columns  
SELECT * FROM tblemp; {Selects all rows and columns}  
SELECT * FROM tblemp; /*Selects all columns and rows*/copy to the clipboard 

 

Summary

In this article on SQL Comment, we have explained the use of a comment in SQL with the three-way to apply comment in SQL that is a single line, multi-line, and inline comment with syntax and practical example also covered nested comment and comment indicators with practical examples

 

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