在Mac Os下通过Gem安装Mysql时，老是报错：

Building native extensions.  This could take a while...
ERROR:  Error installing mysql:
	ERROR: Failed to build gem native extension.

        /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby extconf.rb
checking for mysql_query() in -lmysqlclient... no
checking for main() in -lm... yes
checking for mysql_query() in -lmysqlclient... no
checking for main() in -lz... yes
checking for mysql_query() in -lmysqlclient... no
checking for main() in -lsocket... no
checking for mysql_query() in -lmysqlclient... no
checking for main() in -lnsl... no
checking for mysql_query() in -lmysqlclient... no
checking for main() in -lmygcc... no
checking for mysql_query() in -lmysqlclient... no

 貌似是缺少了某些文件导致的，于是看了看/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby extconf.rb发现extconf.rb好像需要Mysql的PATH，于是执行：

export PATH=$PATH:/usr/local/mysql/bin
sudo gem install mysql -v '2.8.1'

问题解决：

Building native extensions.  This could take a while...
Successfully installed mysql-2.8.1
1 gem installed
Installing ri documentation for mysql-2.8.1...
Installing RDoc documentation for mysql-2.8.1...