MySQL - Comments

The Comments are added in a MySQL statement with the purpose of making the statement easier to understand. It makes the MySQL statement more readable and hence easier to update it later. Comments are ignored when the statement is executed. In MySQL, there are two ways of putting a comment.

Single line comment
Multi-line comment

Consider a database table called Employee with the following records:

EmpID	Name	City	Age	Salary
1	John	London	25	3000
2	Marry	New York	24	2750
3	Jo	Paris	27	2800
4	Kim	Amsterdam	30	3100
5	Ramesh	New Delhi	28	3000
6	Huang	Beijing	28	2800

Single line Comment

In MySQL, a single line comment can be done by two ways:

using # character : Starts with # and ends with the end of that line.

using -- : Starts with -- followed by at least one whitespace or control character (such as a space, tab and so on) and ends with the end of that line.

These single line comments will be ignored when the statement is executed.

Example:

In the example below, two line comments are used. Comments are ignored when a statement is executed.

-- first line comment
#second line comment
SELECT Name, Salary FROM Employee; -- third line comment

This will produce the result as shown below:

Name	Salary
John	3000
Marry	2750
Jo	2800
Kim	3100
Ramesh	3000
Huang	2800

Multi-line Comment

It starts with /* and ends with */. Anything between /* and */ is a block comment and will be ignored when the statement is executed.

Example:

In the below MySQL statement, multi-line comments are used. When this statement is executed these block of comments will be ignored.

/* comment line 1
comment line 2 */
SELECT EmpID, Name, City  /* more comments */  FROM Employee;

This result of the above MySQL statement will be:

EmpID	Name	City
1	John	London
2	Marry	New York
3	Jo	Paris
4	Kim	Amsterdam
5	Ramesh	New Delhi
6	Huang	Beijing