安裝MySQL客戶端出現：
# apt-get install mysql-client
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package mysql-client is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package 'mysql-client' has no installation candidate
安裝MySQL服務出現
# apt-get install mysql-server
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package mysql-server is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package 'mysql-server' has no installation candidate
參考：install mysql - has no installation candidate
# apt-cache search mysql-server
mariadb-server-10.3 - MariaDB database server binaries
mariadb-server-core-10.3 - MariaDB database core server files
default-mysql-server - MySQL database server binaries and system database setup (metapackage)
default-mysql-server-core - MySQL database server binaries (metapackage)
看到輸出的內容，就知道怎麼安裝了吧？