这解决了我在Windows中的问题:

subst X: "C:\Program files\MySQL\MySQL Server 5.5" 
gem install mysql2 -v 0.x.x --platform=ruby -- --with-mysql-dir=X: --with-mysql-lib=X:\lib\opt 
subst X: /D

如果您正在使用yum，请尝试:

sudo yum install mysql-devel

MacOS用户的另一种方式

如果你使用“brew”安装mysql:

gem install mysql2 -v 'x.x.x' -- --with-mysql-config=/usr/local/Cellar/mysql/y.y.y/bin/mysql_config

X.X.X =要安装的mysql2 gem的版本号 y.y.y =你安装的mysql版本ls /usr/local/Cellar/mysql

你遇到这个问题是因为你没有安装MySql。在安装mysql2 gem之前。安装MySQL。之后将安装mysql2 gem。

在Debian Stretch上，适合我的包是default-libmysqlclient-dev

Sudo apt-get update && apt-get install -y default-libmysqlclient-dev

根据https://github.com/brianmario/mysql2/issues/1175，我用

gem install mysql2 -- \
 --with-mysql-lib=/usr/local/Cellar/mysql/8.0.26/lib \
 --with-mysql-dir=/usr/local/Cellar/mysql/8.0.26 \
 --with-mysql-config=/usr/local/Cellar/mysql/8.0.26/bin/mysql_config \
 --with-mysql-include=/usr/local/Cellar/mysql/8.0.26/include