MySQL dump file and commented out lines

Question

I have these lines at the top of a MySQL .sql file (dumped with phpMyAdmin):

/*!40101 SET @OLD_CHARACTER_SET_CLIENT=@@CHARACTER_SET_CLIENT */; /*!40101 SET @OLD_CHARACTER_SET_RESULTS=@@CHARACTER_SET_RESULTS */; /*!40101 SET @OLD_COLLATION_CONNECTION=@@COLLATION_CONNECTION */; /*!40101 SET NAMES utf8 */; 

But they all seem to be commented out. So will MySQL ignore them when the file is loaded back into a database? If so, why generate these lines at all?

Answer 1

These are conditional comments aimed at certain versions of mySQL.

From here and here:

• MySQL version specific comments start with /*!
• And end with */
• Version numbers are always 5 digits
• Version numbers are in the format major release number, minor release number, revision number with the major being 1 digit and the minor and revision being 2 digits left-padded with 0’s
• Version specific comments will target any version equal to or higher than the version number listed

Answer 2

MySQL will parse and execute these lines of code, I believe for localization and character-encoding stuff, because the comments start with /*! (with the exclamation mark) instead of just the C-style /*.

Think of it the same way you do conditional comments and Internet Explorer. <!--[if IE]><![endif]--> looks like any plain old HTML comment to other browsers, but IE recognizes this special comment and parses it as needed. Likewise, /*! */ looks like any other comment to other SQL DBMSes but is special to MySQL.