ubuntu12.04中将mysql升级到5.5.22导致原先的mysql加载失败

解决方法：

1.完全卸载原先的mysql

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sudo apt-get remove mysql-server
sudo apt-get autoremove --purge mysql-server
sudo apt-get remove mysql-common(这步比较重要)
dpkg -l | grep ^rc | awk '{print $2}' | sudo xargs dpkg -P

2.重新安装mysql（安装前最好重启）

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sudo apt-get install mysql-server
sudo apt-get install mysql-workbench(原先的client无法使用)

可使用以下命令检查是否安装成功

sudo netstat -tap | grep mysql

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3原先gem中的mysql2需要重新编译安装

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gem uninstall mysql2
gem install mysql2

4.安装mysql2时可能出现以下错误

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Gem::Installer::ExtensionBuildError: ERROR: Failed to build gem native extension.

/homexxx/.rbenv/versions/1.9.3-p125/bin/ruby extconf.rb 
checking for rb_thread_blocking_region()... yes
checking for rb_wait_for_single_fd()... yes
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... yes
checking for mysql_query() in -lmysqlclient... no
checking for main() in -lmygcc... no
checking for mysql_query() in -lmysqlclient... no
*** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of
necessary libraries and/or headers.  Check the mkmf.log file for more
details.  You may need configuration options.

?解决方法：

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sudo apt-get install libmysql-ruby
sudo apt-get install libmysqlclient-dev
gem install mysql