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In this article, we will introduce MySQL comments. We will also understand what type of comments should be used and where.

Comments are written to describe the code and make it easier to understand. We also use comments to ignore a part of the code (do not let that part of the code be executed) while parsing SQL queries.

It also helps other programmers understand what is happening in your code. We will see how to use single and multi-line comments in MySQL. We will also learn about executable comments and their uses.

There are three different ways to write MySQL comments. We can use #, , -or /*, and */symbols to comment.

MySQL single line comments

We can use MySQL single-line comments in two ways, using #or -.

Let us create a tb_teacherstable called to practice MySQL comments. We use #to write single line comments in the MySQL query given below.

This comment is used at SQLthe end of the query and must be followed by a newline. If you do not use a newline after the comment, everything on the same line will be commented until a newline is encountered. It is not necessary #to put a space after the query. However, #it is a good idea to put a space after the query to increase readability.

#Example code using MySQL:

# Following SQL query will create a table named 'tb_teachers'
CREATE TABLE `practice_mysql_comments`.`tb_teachers` (
 TEACHER_ID	INTEGER NOT NULL,
 FIRST_NAME	VARCHAR(30) NOT NULL,
 LAST_NAME	VARCHAR(30) NOT NULL,
 GENDER	VARCHAR(30) NOT NULL,
 CITY	VARCHAR(64) NOT NULL,
 EMAIL	VARCHAR(64) NOT NULL,
 JOINING_YEAR INTEGER NOT NULL,
 PRIMARY KEY	(TEACHER_ID)
);

See the following code example where we do not put a newline after using single line comment. You can see that CREATEthe command is also partially commented.

# Following SQL query will create a table named 'tb_teachers' CREATE TABLE `practice_mysql_comments`.`tb_teachers` (
 TEACHER_ID	INTEGER NOT NULL,
 FIRST_NAME	VARCHAR(30) NOT NULL,
 LAST_NAME	VARCHAR(30) NOT NULL,
 GENDER	VARCHAR(30) NOT NULL,
 CITY	VARCHAR(64) NOT NULL,
 EMAIL	VARCHAR(64) NOT NULL,
 JOINING_YEAR INTEGER NOT NULL,
 PRIMARY KEY	(TEACHER_ID)
);

We can use this type of comments in SQL queries as well. Refer to the following code; you can observe that we can use <pre> #to comment in SQL queries.

# Following SQL query will create a table named 'tb_teachers' 
CREATE TABLE `practice_mysql_comments`.`tb_teachers` (
 TEACHER_ID	INTEGER NOT NULL, # teacher's id
 FIRST_NAME	VARCHAR(30) NOT NULL, # teacher's first name
 LAST_NAME	VARCHAR(30) NOT NULL, # teacher's last name
 GENDER	VARCHAR(30) NOT NULL, # teacher's gender
 CITY	VARCHAR(64) NOT NULL, # teacher's home town
 EMAIL	VARCHAR(64) NOT NULL, # teacher's email
 JOINING_YEAR INTEGER NOT NULL, # teacher's appointment year
 PRIMARY KEY (TEACHER_ID) # primay key of the teacher's table
);

Let us explore another way of single line commenting using the _ symbol. The following SQL code shows that we can use the double (dash) symbol for commenting -even in SQL queries .-

This is the same as we use #the em dash symbol for comments, with one difference. We must put at least one space after the second dash; otherwise, it will not work as a comment.

-- Following SQL query will create a table named 'tb_students' 
CREATE TABLE `practice_mysql_comments`.`tb_students` (
 STUDENT_ID	INTEGER NOT NULL, -- student's id
 FIRST_NAME	VARCHAR(30) NOT NULL, -- student's first name
 LAST_NAME	VARCHAR(30) NOT NULL, -- student;s last name
 GENDER	VARCHAR(30) NOT NULL, -- student's gender
 CITY	VARCHAR(64) NOT NULL, -- student's home town
 EMAIL	VARCHAR(64) NOT NULL, -- student's email
 REGISTRATION_YEAR INTEGER NOT NULL, -- student's registration year
 PRIMARY KEY (STUDENT_ID) -- primay key of the student's table
);

MySQL multiple rows注释

If we need to explain the SQL query in detail to make the code easy to understand, we use multi-line comments. /* */Everything inside will be ignored. It does not matter whether you put a line break at the end of such a comment, but it is better to have one to write clean and organized code.

/* 
Following SQL query will create a table named 'tb_students'
having the basic information about the students. This information includes
full name, gender, city, email and registration year.
*/
CREATE TABLE `practice_mysql_comments`.`tb_students` (
 STUDENT_ID	INTEGER NOT NULL, 
 FIRST_NAME	VARCHAR(30) NOT NULL, 
 LAST_NAME	VARCHAR(30) NOT NULL, 
 GENDER	VARCHAR(30) NOT NULL, 
 CITY	VARCHAR(64) NOT NULL, 
 EMAIL	VARCHAR(64) NOT NULL,
 REGISTRATION_YEAR INTEGER NOT NULL, 
 PRIMARY KEY (STUDENT_ID) 
);

Although we can treat multi-line comments as single line, but why do we have single line comments and add more work. It is better to use multi-line comments when you don't want to execute more than one line. See the following example.

/* 
Following SQL query will create a table named 'tb_students'
having the basic information about the students. This information includes
full name, gender, city, email and registration year.
*/
CREATE TABLE `practice_mysql_comments`.`tb_students` (
 STUDENT_ID	INTEGER NOT NULL, 
 FIRST_NAME	VARCHAR(30) NOT NULL, 
 LAST_NAME	VARCHAR(30) NOT NULL, 
 GENDER	VARCHAR(30) NOT NULL, 
 CITY	VARCHAR(64) NOT NULL, 
 /* EMAIL	VARCHAR(64) NOT NULL,
 REGISTRATION_YEAR INTEGER NOT NULL, */
 PRIMARY KEY (STUDENT_ID) 
);

MySQL executable file comments

MySQL also supports executable comments. This type of comment provides you with portability between various databases. The code written in these comments will only be executed in MySQL. If you !specify a version character after , it will only work with that specific MySQL version or higher.

Example code for executable annotations:

SELECT 3 /*! *2 */ AS MULTIPLICATION;

in conclusion

We have concluded that MySQL comments play a vital role while programming. It does not matter what type of programming it is. If you have to ignore multiple lines, using multi-line comments is a good choice; otherwise, single-line comments are fine. If you want a certain section of code to be executed only on the MySQL server, then you can use MySQL executable comments.