-- 合并查询 union all以及union
-- union all就是将两个查询结果合并,不会去重
select ename,sal,job from emp where sal>2500; -- 5条记录
select ename,sal,job from emp where job MANAGER -- 3条记录select ename,sal,job from emp where sal>2500
un…
集合类不安全
list不安全 //报错 java.util.ConcurrentModificationException
public class ListTest {public static void main(String[] args) {List<String> list new CopyOnWriteArrayList<>();//并发下Arrayist边读边写会不安全的/*** 解决方案:…