用户将不能从其他数据库角色继承权限，因为 WHERE 子句基于用户名或登录名而不基于角色。
Users will not be able to inherit permissions from other database roles, because the WHERE clause is based on user or login names, not on roles.
实现Runnable的类可以不继承Thread而实例化为Thread实例，将自身作为运行目标。
A class that implements Runnable can run without subclassing Thread by instantiating a Thread instance and passing itself in as the target.
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