linux apt-get install no candidate packages，but apt-get remove working normal！

I install bash-completion by apt-get install bash-completion,and then I set source /etc/share/bash-completion/completion in ~/.bashrc ,apt-get inst + tab will become apt-get install ,but when I type apt-get install mysql+tab ,no candidate package for example mysql-server ;I hava run apt-get update before!I am so depressed!can you give me some suggestion!

coment：apt-get remove work normal!

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