PLSQL基础知识

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       )