SQL: 结构化查询语言
dual:虚表，伪表，用于补齐语法结构
select 1+1 from dual;
别名查询：
select ename 姓名,sal 工资 from emp;去除重复数据：(一列都一样才算重复）
select distinct job from emp;sql四则运算：
select 1+1 from dual;
select sal*12 from emp;
select sal*12 + nvl(comm,0) from emp;
nvl()函数，如果第一个参数为空，则返回第二个参数
注意：包含null的表达式都为null；空值（未知，不可预测的值）永远不等于空值
连接符: ||
mysql形式：
select concat(ename,‘的工资是‘,sal,) from emp;oracle:
select ‘工资：‘ || sal from emp;where语句：
select * from emp where comm is null;
select * from emp where comm is not null;select * from emp where sal >1000;
select * from emp where comm is null and not(sal>1000);
select * from emp where sal between 1000 and 2000;
select * from emp where sal in (1000,3000);
select * from emp where comm is not null or sal >1800;select * from emp where ename like ‘李%‘;
select * from emp where ename like ‘李_‘;排序查询：
order by (asc：升序；desc:降序，默认升序）
select * from emp order by eid;
select * from emp order by eid desc;遇到空值问题时，利用nulls first 或 nulls last 来解决
select * from emp order by sal desc nulls last;sql的函数：
D:\BaiduNetdiskDownload\oracle\day1\源码\预习（本机资源）
sql的常用函数参考:https://www.cnblogs.com/canyangfeixue/p/3203588.html
注意：where 和having的最大区别，where后面不能跟组函数
错误示范：
select deptno avg(sal) from emp group by deptno where avg(sal)>3000;
select sal deptno,count(ename) from emp group by deptno;
select sal deptno,count(ename) from emp;原文：https://www.cnblogs.com/wuba/p/11025947.html