MySql Comment Syntax - What's the difference between “#” and “— ”

Question

According to the documentation , there are two ways you can comment to the end of a line in MySql:

• "#"
• "-- "

Is there any difference between these?

If so, when should I use one vs. the other?

If not, does anyone know why both are supported?

Seems strange to me, especially when the hyphenated version still differs slightly from the standard SQL Syntax .

Answer 1

It only really matters if you want your SQL to be portable. For instance -- comments are OK in Sqlite and Postegresql while # are not. Your best bet is to use -- with a space following. (As far as I can remember I've hardly ever seen anything else)

Answer 2

As the link you provided clearly explain it -- is the standard SQL "comment separator". Where MySQL departs from standard is in requiring an space after -- to be recognized as a comment. "Standard" SQL does not require this.

To provide an example, in the following code, -- is recognized as a comment token:

mysql> CREATE TABLE T(C int);   -- This is my new table
Query OK, 0 rows affected (0.18 sec)

But notice how the interactive interpreter misbehave without space after -- :

mysql> CREATE TABLE T(C int);   --This is my new table
Query OK, 0 rows affected (0.24 sec)

    -> ;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '--This is my new table' at line 1

---

MySQL support some other comment format to accommodate with habit of various programmers : # like many script language and /* ... */ like C. It is quite astounding that // is not yet part of them.

mysql> CREATE TABLE T(C int);   /* This is my new table */
Query OK, 0 rows affected (0.22 sec)

mysql> CREATE TABLE T(C int);   # This is my new table
Query OK, 0 rows affected (0.24 sec)

Answer 3

Both Oracle and SQL Server (as well as DB2 and others) support the -- and /* */ (multi-line) comment standard. So it's a good habit to use the same style in MySQL, as you will probably encounter situations where you're programming in multiple environments.

The requirement for a space after -- is a MySQL oddity, but is hardly a big deal as it helps clarity.

If, of course, you are using a lot of perl and MySQL, then you may opt for the # comment for similar reasons...