PLSQL语言（一）

第一课：客户端
        1. Sql Plus(客户端），命令行直接输入：sqlplus，然后按提示输入用户名，密码。
        2. 从开始程序运行:sqlplus，是图形版的sqlplus.
        3. http://localhost:5560/isqlplus
        
        Toad：管理， PlSql Developer:


第二课：更改用户
        1. sqlplus sys/bjsxt as sysdba
        2. alter user scott account unlock;(解锁)

第三课：table structure
        
        1. 描述某一张表：desc 表名
        2. select * from 表名
第四课：select 语句：

       1.计算数据可以用空表：比如：.select 2*3 from dual

       2.select ename,sal*12 annual_sal from emp;与select ename,sal*12 "annual sal" from emp;区别，加双引号保持原大小写。不加全变大写。
       
       3. select ename || "abcd" 如果连接字符串中含有单引号，用两个单引号代替一个单引号。

第五课：distinct
        select deptno from emp;
        select distinct deptno from emp;

        select distinct deptno from emp;
        select distinct deptno ,job from emp
        去掉deptno,job两者组合的重复。更多的项，就是这么多项的组合的不重复组合。
第六课：Where
        select * from emp where deptno =10;
        select * from emp where deptno <>10;不等于10        
        select * from emp where ename ='bike';
        select ename,sal from emp where sal between 800 and 1500 (>=800 and <=1500)
        空值处理:
        select ename,sal,comm from emp where comm is (not) null;
        select ename,sal,comm from emp where ename ( not)in ('smith','king','abc');
        select ename from emp where ename like '_A%';_代表一个字母,%代表0个或多个字母. 如果查询%
        可用转义字符.\%. 还可以用escape '$'比如:select ename from emp where ename like '%$a%' escape '$';

第七课: orderby
        
         select * from dept; 
         select * from dept order by dept desc;(默认:asc)
         select ename,sal,deptno from emp order by deptno asc,ename desc;

第八课: sql function1:    
        select ename,sal*12 annual_sal from emp
        where ename not like '_A%' and sal>800
        order by sal desc;

        select lower(ename) from emp;

        select ename from emp 
        where lower(ename) like '_a%';等同于
        select ename from emp where ename like '_a%' or ename like '_A%';

        select substr(ename,2,3) from emp;从第二字符截,一共截三个字符.
        select chr(65) from dual 结果为:A
        select ascii('a') from dual 结果为:65
        select round(23.652,1) from dual; 结果为: 23.7
        select round(23.652,-1) from dual; 20
  
        select to_char(sal,'$99_999_999') from emp;
        select to_char(sal,'L99_999_999') from emp;人民币符号,L:代表本地符号

        这个需要掌握牢:
        select birthdate from emp;
        显示为:
        BIRTHDATE
        ----------------
        17-12月-80
        ----------------

        改为:
        select to_char(birthdate,'YYYY-MM-DD HH:MI:SS') from emp;
        
        显示:
         
        BIRTHDATE
        -------------------
        1980-12-17 12:00:00
        -------------------
        
        select to_char(sysdate,'YYYY-MM-DD HH24:MI:SS') from dual; //也可以改为:HH12
        TO_CHAR(SYSDATE,'YY
        -------------------
        2007-02-25 14:46:14
        

        to_date函数:
        select ename,birthdate from emp where birthdate > to_date('1981-2-20 12:34:56','YYYY-MM-DD HH24:MI:SS');
        如果直接写 birthdate>'1981-2-20 12:34:56'会出现格式不匹配,因为表中的格式为: DD-MM月-YY.
  
        
        select sal from emp where sal>888.88 无错.但
        select sal from emp where sal>$1,250,00;
        会出现无效字符错误. 
        改为:
        select sal from emp where sal>to_number('$1.250.00','$9,999,99');
        
        把空值改为0
        select ename,sal*12+nvl(comm,0) from emp;
        这样可以防止comm为空时,sal*12相加也为空的情况.


第九课: Group function 组函数
        max,min,avg ,count,sum函数
        
        select to_char(avg(sal),'99999999,99') from emp;
        
        
        select round(avg(sal),2) from emp;
        结果:2073.21
          
        select count(*) from emp where deptno=10;
        select count(ename) from emp where deptno=10; count某个字段,如果这个字段不为空就算一个.
        select count(distinct deptno) from emp;
        select sum(sal) from emp;

第十课: Group by语句
        
        需求:现在想求,求每个部门的平均薪水.
        select avg(sal) from emp group by deptno;
        select deptno avg(sal) from emp group by deptno;
        
        select deptno,job,max(sal) from emp group by deptno,job;
        
       求薪水值最高的人的名字.
       select ename,max(sal) from emp;出错,因为max只有一个值,但等于max值的人可能好几个,不能匹配.
       应如下求:
       select ename from emp where sal=(select max(sal) from emp);

       Group by语句应注意,

       出现在select中的字段,如果没出现在组函数中,必须出现在Group by语句中.
        
    
  第十一课: Having 对分组结果筛选
       
       Where是对单条纪录进行筛选,Having是对分组结果进行筛选.
      
       select avg(sal),deptno from emp 
       group by deptno 
       having avg(sal)>2000;
       
       查询工资大于1200雇员,按部门编号进行分组,分组后平均薪水大于1500,按工薪倒充排列.
       select * from emp 
       where sal>1200
       group by deptno
       having avg(sal)>1500
       order by avg(sal) desc;
        
   第十二课:字查询
       
       谁挣的钱最多(谁:这个人的名字,  钱最多)
       
       select 语句中嵌套select 语句,可以在where,from后.
       
             
       问那些人工资,在平均工资之上.
       
       select ename,sal from emp where sal>(select avg(sal) from emp);


       查找每个部门挣钱最多的那个人的名字.
       select ename ,deptno from emp where sal in(select max(sal) from ename group by deptno) 查询会多值.

       应该如下:
       
       select  max(sal),deptno from emp group by deptno;当成一个表.语句如下:
       select ename, sal from emp join(select  max(sal) max_sal,deptno from emp group
       by deptno) t on(emp.sal=t.max_sal and emp.deptno=t.deptno); 
       
       每个部门的平均薪水的等级. 
       分析:首先求平均薪水(当成表),把平均薪水和另外一张表连接.
       
       
第十四课:self_table_connection
       
       把某个人的名字以及他的经理人的名字求出来(经理人及这个人在表中同处一行)
       
       分析:首先求出这个人的名字,取他的编号,然后从另一张表与其相对应编号,然后找到经理的名字.
       
       select e1.ename ,e2.ename from emp e1,emp e2 where e1.mgr= e2.empno.
       
       empno编号和MGR都是编号.


第十15课: SQL1999_table_connections    
         
      select ename,dname,grade from emp e,dept d, sqlgrade s
      where e.deptno = d.deptno and e.sql between s.losal and s.hisal and
      job<>'CLERK';
      
      有没有办法把过滤条件和连接条件分开来? 出于这样考虑,Sql1999标准推出来了.有许多人用的还是
      旧的语法,所以得看懂这种语句.
      
      
      
      select ename,dname from emp,dept;(旧标准).
      select ename,dname from emp cross join dept;(1999标准)
       
      select ename,dname from emp,dept where emp.deptno=dept.deptno (旧) 
      select ename,dname from emp join dept on(emp.deptno = dept.deptno); 1999标准.没有Where语句.
      select ename,dname from emp join dept using(deptno);等同上句,但不推荐使用.
      
      select ename,grade from emp e join salgrade s on(e.sal between s.losal and s.hisal);
      join 连接语句, on过滤条件。连接，条件一眼分开。如果用Where语句较长时，连接语句和过滤语句混在一起。
      
      三张表连接：
      slect ename,dname, grade from 
      emp e join dept d on(e.deptno=d.deptno)
      join salgrade s on(e.sal between s.losal and s.hisal)
      where ename not like '_A%';
      把每张表连接 条件不混在一起，然后数据过滤条件全部区分开来。读起来更清晰，更容易懂一点。
      
      select e1.ename,e2.ename from emp e1 join emp e2 on(e1.mgr = e2.emptno);

      左外连接：会把左边这张表多余数据显示出来。
      select e1.ename,e2,ename from emp e1 left join emp e2 on(e1.mgr =e2.empno);left 后可加outer
      右外连接：
    select ename,dname from emp e right outer join dept d on(e.deptno =d.deptno); outer可以取掉。
        
      即把左边多余数据，也把右边多余数据拿出来，全外连接。
      select ename,dname from emp e full join dept d on(e.deptno =d.deptno); 


16-23 课：求部门平均薪水的等级

       A.求部门平均薪水的等级。

       select deptno,avg_sal,grade from 
       (select deptno,avg(sal) avg_sal from emp group by deptno)t
       join salgrade s on(t.avg_sal between s.losal and s.hisal)
       
       B.求部门平均的薪水等级
       select deptno,avg(grade) from 
       (select deptno,ename, grade from emp join salgrade s on(emp.sal between s.losal and
       s.hisal)) t
       group by deptno

       C.那些人是经理
       select ename from emp where empno in(select mgr from emp);
       select ename from emp where empno in(select distinct mgr from emp);
       
       D.不准用组函数，求薪水的最高值（面试题）
       
       select distinct sal from emp where sal not in(
       select distinct e1.sal from emp e1 join emp e2 on (e1.sal<e2.sal));
       
       E.平均薪水最高的部门编号
       
       select deptno,avg_sal from
       (select avg(sal)avg_sal,deptno from emp group by deptno)
       where avg_sal=
       (select max(avg_sal)from 
       (select avg(sal) avg_sal,deptno from emp group by deptno)
       )
      
       F.平均薪水最高的部门名称
       select dname from dept where deptno=
      ( 
        select deptno from
        (select avg(sal)avg_sal,deptno from emp group by deptno)
        where avg_sal=
        (select max(avg_sal)from 
        (select avg(sal) avg_sal,deptno from emp group by deptno)
        )
       )
      
      G.求平均薪水的等级最低的部门的部门名称
        
        组函数嵌套
        如：平均薪水最高的部门编号，可以E.更简单的方法如下：
        select deptno,avg_sal from 
        (select avg(sal) avg_sal,deptno from emp group by deptno)
        where avg_sal =
        (select max(avg(sal)) from emp group by deptno)
        
        组函数最多嵌套两层
        
        分析：
        首先求
        1.平均薪水： select avg(sal) from group by deptno;

        2.平均薪水等级：  把平均薪水当做一张表，需要和另外一张表连接salgrade
        select  deptno,grade avg_sal from 
          ( select deptno,avg(sal) avg_sal from emp group by deptno) t
        join salgrade s on(t.avg_sal between s.losal and s.hisal)
        
        上面结果又可当成一张表。
        
        DEPTNO    GRADE    AVG_SAL
      --------  -------  ----------
        30           3   1566.66667
        20           4   2175
        10           4   2916.66667

        3.求上表平均等级最低值
        
        select min(grade) from
        (
          select deptno,grade,avg_sal from
           (select deptno,avg(sal) avg_sal from emp group by deptno)t
          join salgrade s on(t.avg_sal between s.losal and s.hisa)
         )

        4.把最低值对应的2结果的那张表的对应那张表的deptno, 然后把2对应的表和另外一张表做连接。
          
          select dname ,deptno,grade,avg_sal from
            (
  	      select deptno,grade,avg_sal from
              (select deptno,avg(sal) avg_sal from emp group by deptno)t
             join salgrade s on(t.avg_sal between s.losal and s.hisal)
             ) t1
            join dept on (t1.deptno = dept.deptno)
            where t1.grade =
            ( 
              select deptno,grade,avg_sal from
               (select deptno,avg(sal) avg_sal from emp group by deptno) t
                join salgrade s on(t.avg_sal between s.losal and s.hisal)
               )
            )
         结果如下：
         
        DNAME    DEPTNO     GRADE    AVG_SAL
      --------  -------  --------   --------
        SALES        30        3    1566.6667 
     
         
       H: 视图（视图就是一张表，一个字查询）
        
       G中语句有重复，可以用视图来简化。
       conn sys/bjsxt as sysdba;
       grant create table,create view to scott;
       conn scott/tiger
       创建视图：
       create view v$_dept_avg-sal_info as
       select deptno,grade,avg_sal from
        ( select deptno,avg(sal) avg_sal from emp group by deptno)t
       join salgrade s on 9t.avg_sal between s.losal and s.hisal)
      
       然后 
       select * from v$_dept_avg-sal_info
       
       结果如下：
       DEPTNO      GRADE    AVG_SAL
      --------  -------  ----------
        30           3   1566.66667
        20           4   2175
        10           4   2916.66667

       然后G中查询可以简化成：
       select  dname,t1.deptno,grade,avg_sal from
       v$_dept_avg-sal_info t1
       join dept on9t1.deptno =dept.deptno)
       where t1.grade=
       (
	select min(grade) from v$_dept_avg-sal_info t1
       )