一、访问方式：
????? (1)命令行
??? ??(2)sqlplus客户端
????? (3)浏览器(http://127.0.0.1:5560/isqlplus/，如果不能访问，则isqlplusctl start)
二、用户解锁：
???? ?登录：sqlplus sys/密码 as sysdba;
????? 解锁：alert user scott account unlock;
三、sql语言：????
??? ? sql1992 structured query language 结构化查询语言
???? ?sql1999两套不同的语言
????? 机器(第一代语言)，汇编(第二代语言)，java c++(第三代语言)? sql语言(第四代语言，无条件，循环，分支)
四、scott中的表
???? emp(雇员信息): comm津贴? job工种? mgr经理人?

???? dept(部门信息)
???? salgrade(薪水等级信息)：grade local hisal

五、select语句

????? 1、基本的sql语句

?????????

             select *from emp;
             select ename,sal*12 from emp;
              dual表:只有一个字段
         select 2*3 from dual:算出的只有一个结果
         select sysdate from dual：系统日期
         select ename, sal*12 anuual_sal from emp;
             //起别名，别名中有特殊字符，则加双引号，双引号保持住原有的格式
         select enmae,comm from emp;好多人没有津贴，0和空值不一样。0是有值
         select ename,sal*12+comm from emp;//算出有的是空值，任何含有空值的表达式结果都是空值
         select ename||sal from emp; 相当于java中的字符串+
             select ename||'asfasdfasdf' from emp;
             select ename|| 'asfasdfa''asdfasdf' from emp 正确语句
         select distinct deptno from emp;
             select distinct deptno,job from emp; 这两个在一起的组合是重复的，就把它去掉

??? 2、带where的sql语句

??????????

      select *from emp where deptno=10;
      select *from emp where ename='CLARK'
      select ename,sal from emp where sal>1500;
      select ename,sal,deptno from emp where deptno<>10;不等于
    select ename,sal from emp where ename>'CBA'
      select ename,sal from emp where sal between 800 and 1500;
      select ename,sal from emp where sal>= 800 and sal<1500;
      select ename,sal,comm from emp where comm is (not) null;
      select ename,sal comm from emp where sal in (800,1500,2000);
      select ename,sal,comm from emp where ename in ('smith','king','abc');
      select ename,sal,hiredate from emp where hiredate>'20-2月-81';
      select ename,sal from emp where deptno=10 and sal>1000;      
      select ename,sal from emp where deptno=10 or sal>1000; 
      select ename,sal from emp where sal not in (800,1500);
      select ename from emp where ename like '%ALL%'; %多个
    select ename from emp where ename like '_A%';  _一个
    select ename from emp where ename like '%\%%'; 转义字符
    select ename from emp where ename like '%$%%' escape '$'; //$是转义字符
    select *from dept;//见这个表的顺序
    select *from dept order by deptno desc;
      select empno,ename from emp order by empno asc;不写默认按升序排列
    select empno,enmae form emp where deptno<>10 order by empno asc;
      select ename,sal,deptno from emp order by deptno asc,ename desc; deptno相同的情况下排列ename
      select ename,sal*12 annal_sal form emp where ename not like '_A%' and sal>800 order by sal desc;

?? 3、sql函数

????????? 3.1单行函数

???????????

select lower(ename) from emp;
select ename from emp where lower(emane) like '_a%' 
select ename from emp where ename like '_a%' or ename like '_A%';
select substring(ename,1,3) from emp 
select char(65) from dual;//acsci转换为字符
select asci('A') form dual;
select round(23.652) from dual 24
select round(23.652,2) from dual 23.65
select round(23.652,-1) from dual 20 
select to_char(sal,'$99,999.9999') from emp; 
    //9代表一位数字，没有就不显示
select to_char(sal,'L99,999.9999') from emp;
select to_char(sal,'$00,000.0000') from emp;没有就显示0
select to_char(hiredate,'YYYY-MM-DD HH:MI:SS') from emp;
select to_char(sysdate,'YYYY-MM-DD HH24:MI:SS') from emp;
select ename,hiredate from emp where hiredate>to_date('1981-2-20 12:34:56','YYYY-MM-DD HH24:MI:SS');
select sal from emp where sal>to_number('$1,250.00','$9,999.99');
select