sql 学习

、查看表结构用desc

    desc emp;

2、空表dual，最常用的空表，如：

    select 2 * 4 from dual;

    select sysdate from dual;

3、双引号能保持格式

    如：select sysdate “toDay 日 期” from dual;

4、|| 字符串连接

    如：select 2*3 || 8 from dual;

    select ename || sal from scott.emp;

    select ename || ‘ORACLE’ from scott.emp;

5、单引号，如：select 2 * 2 || 'abc''efg' from dual;

    用两个单引号表示一个单引号

6、去掉重复数据distinct

    select distinct deptno from scott.emp;

    去掉重复组合：select distinct deptno,job from scott.emp;

7、where查询

    A、=查询，select * from scott.emp where sal = 1500;

    B、比较<、>、>=、<=

        select * from scott.emp where sal > 1500;

    C、and or

        select * from scott.emp where sal > 1500 and sal <= 5000 or deptno = 10;

    D、in、not in

        select * from scott.emp where sal in (1500, 800) and deptno not in (10, 20)

    E、like模糊 escape 转义

        Select * from scott.emp where ename like ‘%in%’;

        Select * from scott.emp where ename like ‘%in\%k%’;

        Select * from scott.emp where ename like ‘%in#%k%’ escape ‘#’;

        表示like中的#号是转义字符，相当于\

    F、is null、is not null

    K、    order by

        select sal, ename from scott.emp order by sal;

        select sal, ename from scott.emp order by sal asc;

        select sal, ename from scott.emp order by sal desc;

        select sal, ename from scott.emp where sal > 2000 order by sal desc;

        select sal, deptno, ename from scott.emp order by sal,deptno desc;

8、function

    A、lower、upper、substr

        select lower(‘abcABC’) from dual;

        select upper(‘abcABC’) from dual;

        substr(target, startIndex, length)

        select substr(‘abcABC’, 1, 3) from dual;

    B、chr、ascii

        将数字安装ascii值转换成字符：select char(65) from dual;

        将字符转换成ascii值：select ascii(‘Z’) from dual;

    C、round、to_char

        精确小数

        select round(22.456) from dual;

        保留2位小数：select round(22.456, 2) from dual;

        精确到个位：select round(22.456, -1) from dual;

        货币

        设置货币格式，000前面不足就用0代替

        select to_char(sal, '$000,000.00') from scott.emp;

        999就不会替换不足的地方，只会安装格式输出

        select to_char(sal, '$999,999.99') from scott.emp;

        本地货币格式

        select to_char(sal, 'L999,999.99') from scott.emp;

        日期

        日期格式 

        格式控制 描述 

        YYYY、YYY、YY 分别代表4位、3位、2位的数字年 

        YEAR 年的拼写 

        MM 数字月 

        MONTH 月的全拼 

        MON 月的缩写 

        DD 数字日 

        DAY 星期的全拼 

        DY 星期的缩写 

        AM 表示上午或者下午 

        HH24、HH12 12小时制或24小时制 

        MI 分钟 

        SS 秒钟 

        SP 数字的拼写 

        TH 数字的序数词 

        “特殊字符” 假如特殊字符 

        HH24:MI:SS AM 15:43:20 PM

        select to_char(sysdate, 'YYYY-MM-DD HH:MI:SS') from dual;

        select to_char(sysdate, 'YYYY-MM-DD HH24:MI:SS') from dual;

     D、to_date、to_number、nvl

        to_date(target, current_format)

        select to_date('2011-4-2 17:55:55', 'YYYY-MM-DD HH:MI:SS') from dual;

        select to_number('$12,322.56', '$999,999.99') + 10 from dual;

        select to_number('$12,322.56', '$00,000.00') + 10 from dual;

        select to_number('22.56') + 10 from dual;

        nvl可以将某个字段的空值转换成指定的值

        select ename, sal, nvl(comm, 1.00) from scott.emp;

9、group function 组函数：min、max、avg、sum、count

        select max(sal) from scott.emp;

        select min(sal) from scott.emp;

        select avg(sal) from emp;

        select round(avg(sal), 2) from emp;

        select to_char(avg(sal), 'L999,999.99') from emp;

        select sum(sal) from emp;

        select count(comm) from emp;

        select count(distinct deptno) from emp;

10、group by 分组

    select deptno, avg(sal) from emp group by deptno;

    select deptno, job, avg(sal) from emp group by deptno, job;

    求部门最高工资的所在部门的员工信息：

    select deptno, ename, sal from emp where sal in (select max(sal) from emp group by deptno);

11、having 对分组数据进行过滤

    求部门评价工资：

    select * from (select avg(sal) sal, deptno from emp group by deptno) where sal > 2000;

    select avg(sal) sal, deptno from emp group by deptno having avg(sal) > 2000;

12、子查询

    求部门分组后工资最高的员工信息

    select emp.ename, emp.sal, emp.deptno from emp, (select max(sal) max_sal, deptno from emp group by deptno) t where emp.sal = t.max_sal and emp.deptno = t.deptno;

    求部门平均工资等级

    select s.grade, t.deptno, t.avg_sal from scott.salgrade s, (select deptno, avg(sal) avg_sal from emp group by deptno) t where t.avg_sal > s.losal and t.avg_sal < s.hisal;（between）

13、自连接

    select a.ename, b.ename mgr_name from emp a, emp b where a.empno = b.mgr;

14、 连接查询

    select dname, ename from dept, emp where dept.deptno = emp.deptno;

    select dname, ename from dept join emp on dept.deptno = emp.deptno;

    select dname, ename from dept join emp using(deptno);

    select dname, ename from dept left join emp on dept.deptno = emp.deptno;

    select dname, ename from dept right join emp on dept.deptno = emp.deptno;

    select dname, ename from dept full join emp on dept.deptno = emp.deptno;

    select a.ename, b.ename mgr_name from emp a join emp b on a.mgr = b.empno;

    select a.ename, b.ename mgr_name from emp a left join emp b on a.mgr = b.empno;

15、 Rownum

    select rounum, deptno, dname from dept;

    select * from (

         select rownum r, dept.* from dept

    ) t where  t.r > 2;

16、树状结构查询

    select level, empno, ename, mgr from emp

    connect by prior mgr = empno;

17、排序函数

    --按部门分组，给出分组后的序号

    select row_number() over(partition by deptno order by sal), emp.* from emp;

    --rank排序，空出相同部分

    select rank() over(partition by deptno order by sal), emp.* from emp;

    select rank() over(order by deptno), emp.* from emp;

    select rank() over(order by sal), emp.* from emp;

    --dense_rank排序给出相同序号,不空留序号

    select rank() over(order by sal), emp.* from emp;

    select dense_rank() over(order by sal), emp.* from emp;

18、交集、并集、割集查询

    --并集：不带重复数据

    select * from emp

    union

    select * from emp2;

    --并集：带重复数据

    select * from emp

    union all

    select * from emp2;        

    --割集,显示不同部分

    select * from emp

    minus

    select * from emp2;

19、 查询系统表、视图

    select owner, object_name, object_type, status, dba_objects.* from dba_objects where object_type = 'view' and status = 'invalid';

    select * from user_objects where object_type like 'PROCEDURE';

20、练习题

    --部门最高薪资员工信息

    select ename, sal, deptno from emp 

    where sal in (select max(sal) from emp group by deptno);

    --部门最高薪资员工信息

    select ename, sal, emp.deptno from emp 

    join (select max(sal) max_sal, deptno from emp group by deptno) t 

    on emp.deptno = t.deptno and emp.sal = t.max_sal;

    --部门平均薪资等级

    select grade, losal, hisal, t.avg_sal from salgrade 

    join (select avg(sal) avg_sal, deptno from emp group by deptno) t

    on t.avg_sal between losal and hisal;

    --经理人

    select ename, job from emp where empno in (select mgr from emp);

    --不用分组函数，查询薪水最高值

    select * from (select sal, ename from emp order by sal desc) where rownum = 1;

    select distinct a.sal from emp a join emp b on a.sal > b.sal where rownum = 1;

    select sal from emp where sal not in (select distinct a.sal from emp a join emp b on a.sal < b.sal);

    --部门平均薪水最高的部门编号

    select deptno, t.avg_sal from (select avg(sal) avg_sal, deptno from emp group by deptno) t

    where avg_sal = (

        select max(avg_sal) max_sal from (select avg(sal) avg_sal, deptno from emp group by deptno)

    );

    select deptno, t.avg_sal from (select avg(sal) avg_sal, deptno from emp group by deptno) t

    where avg_sal = (

        select max(avg(sal)) max_sal from emp group by deptno

    );

    --部门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) t

     where avg_sal = (

            select max(avg_sal) max_sal from (select avg(sal) avg_sal, deptno from emp group by deptno)

     )

    );

    select dname from dept where deptno = (

        select deptno from (select avg(sal) avg_sal, deptno from emp group by deptno) t

        where avg_sal = (

               select max(avg(sal)) from emp group by deptno

        )

    );

    --平均薪水最低的部门的部门名称

    select dname from dept where deptno = (

      select deptno from (select avg(sal) avg_sal, deptno from emp group by deptno) 

      where avg_sal = (

        select min(avg_sal) min_sal from (

               select avg(sal) avg_sal from emp group by deptno

        )

      )

    );

    select dname from dept where deptno = (

        select deptno from (select avg(sal) avg_sal, deptno from emp group by deptno) 

        where avg_sal = (    

          select min(avg(sal)) avg_sal from emp group by deptno

        )

    );

    --平均薪水等级最低的部门的部门名称

    select dname from dept where deptno = (

    select deptno from (

         select grade, t.deptno from salgrade s join (

            select avg(sal) avg_sal, deptno from emp group by deptno

         ) t

         on t.avg_sal between s.losal and s.hisal

      )

      where grade = (

        select min(grade) from salgrade s join (

            select avg(sal) avg_sal, deptno from emp group by deptno

        ) t

        on t.avg_sal between s.losal and s.hisal

      )

    );

    --部门经理人中，平均薪水最低的部门名称

    select t.deptno, dname from (

        select sal, deptno from emp where empno in (select distinct mgr from emp)

    ) t join dept 

    on t.deptno = dept.deptno

    where sal = (

        select min(sal) from emp where empno in (select distinct mgr from emp)

    );

    --比普通员工的最高薪水还要高的经理人名称

    select * from (

        select empno, ename, sal from emp where empno in (select distinct mgr from emp where mgr is not null)

    ) t

    where t.sal > (

        select max(sal) max_sal from emp where empno not in (

         select distinct mgr from emp where mgr is not null

        )

    );