摘要： 下面是一个调用已经私有化的单列的函数的列子. 这里用了静态内部类，关键就是静态内部类可以访问外部类的私有构造函数。这种算是变种继承吧。前提是可以在原来的单列类里添加代码。class Single { private Single(){ System.out.println("Single... 阅读全文

摘要： GROUP BYgroups the result set into summary rows by provided columns. For example, consider below data which contains sales figures by region.GroupName... 阅读全文

摘要： By usingGROUPING SETS()we can specify multiple groupings in a single query.GROUPING SETS()generates the result by producing aUNION ALLset of the resul... 阅读全文