There are three supported comment styles in MariaDB:

* From a '#' to the end of a line:SELECT * FROM users; # This is a comment

* From a '-- ' to the end of a line. The space after the two dashes is
required (as in MySQL).SELECT * FROM users; -- This is a comment

* C style comments from an opening '/*' to a closing '*/'. Comments of this
form can span multiple lines:SELECT * FROM users; /* This is a
multi-line
comment */

Nested comments are possible in some situations, but they are not supported or
recommended.

Executable Comments
-------------------

As an aid to portability between different databases, MariaDB supports
executable comments. These special comments allow you to embed SQL code which
will not execute when run on other databases, but will execute when run on
MariaDB.

MariaDB supports both MySQL's executable comment format, and a slightly
modified version specific to MariaDB. This way, if you have SQL code that
works on MySQL and MariaDB, but not other databases, you can wrap it in a
MySQL executable comment, and if you have code that specifically takes
advantage of features only available in MariaDB you can use the MariaDB
specific format to hide the code from MySQL.

Executable Comment Syntax
-------------------------

MySQL and MariaDB executable comment syntax:

/*! MySQL or MariaDB-specific code */

Code that should be executed only starting from a specific MySQL or MariaDB
version:

/*!##### MySQL or MariaDB-specific code */

The numbers, represented by '######' in the syntax examples above specify the
specific the minimum versions of MySQL and MariaDB that should execute the
comment. The first number is the major version, the second 2 numbers are the
minor version and the last 2 is the patch level.

For example, if you want to embed some code that should only execute on MySQL
or MariaDB starting from 5.1.0, you would do the following:

/*!50100 MySQL and MariaDB 5.1.0 (and above) code goes here. */

MariaDB-only executable comment syntax (starting from MariaDB 5.3.1):

/*M! MariaDB-specific code */
/*M!###### MariaDB-specific code */

MariaDB ignores MySQL-style executable comments that have a version number in
the range 50700..99999. This is needed to skip features introduced in
MySQL-5.7 that are not ported to MariaDB 10.x yet.

/*!50701 MariaDB-10.x ignores MySQL-5.7 specific code */

Note: comments which have a version number in the range 50700..99999 that use
MariaDB-style executable comment syntax are still executed.

/*M!50701 MariaDB-10.x does not ignore this */

Statement delimiters cannot be used within executable comments.

Examples
--------

In MySQL all the following will return 2: In MariaDB, the last 2 queries would
return 3.

SELECT 2 /* +1 */;
SELECT 1 /*! +1 */;
SELECT 1 /*!50101 +1 */;
SELECT 2 /*M! +1 */;
SELECT 2 /*M!50301 +1 */;

The following executable statement will not work due to the delimiter inside
the executable portion:

/*M!100100 select 1 ; */
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual
that corresponds to your MariaDB server version for the right syntax to use
near '' at line 1

Instead, the delimiter should be placed outside the executable portion:

/*M!100100 select 1 */;
+---+
| 1 |
+---+
| 1 |
+---+

URL: https://mariadb.com/kb/en/comment-syntax/