Chapter 1: Simple Query Sentence

Chapter 1: Simple Query Sentence

select * from emp;

select  e.empno, e.ename, e.sal from emp e;
//查询语句中选择指定的列就是关系数据库中所称的投影(Project).

一个完整的SQL命令叫语句(statement),比如 select * from emp;
"select * "和"from emp"叫子句，可以叫做select子句，from子句。

SQL> alter session set NLS_DATE_LANGUAGE = AMERICAN;

Session altered

Sqlplus 默认显示:
1、字符日期左对齐
2、数字右对齐

select  e.empno, e.ename, e.sal*12+500 from emp e;
优先级：
先乘除，在加减
表达式中间同一级从左到右
有括号，括号优先
多层括号，内层括号优先

别名：
select  e.empno AS "Employee Number", e.ename, e.sal*12+500 "Annual Salary" from emp e;

连接运算符：
select e.ename || ' annual salary is ' || (e.sal+500)*12 "Employee's Salary"
from emp e;

select e.ename || ' 年薪为 ' || (e.sal+500)*12 "Employee's Salary"
from emp e;

Distinct 运算符
select  e.deptno
from emp e;

select  distinct e.deptno
from emp e;

select  distinct e.deptno,e.job
from emp e;