sql文练习

select name,sal

case 

 when sal <= 2000 THEN ‘过低‘

 when sal >= 4000 THEN ‘过高‘

 ELSE  ‘OK‘

END AS stats FROM emp WHERE deptno = 10;

SELECT 档次，count(*) AS 人数

    FROM(SELECT (CASE

         WHEN sal<=1000 THEN ‘0000-1000‘

         WHEN sal<=2000 THEN ‘1000-2000‘                                                                           

         WHEN sal<=3000 THEN ‘2000-3000‘

         ELSE ‘好高‘  END)  AS 档次,ename,sal FROM emp)

   GROUP BY 档次

   ORDER BY 1 ;

SELECT * FROM(SELECT rownum AS sn,emp.* FROM emp WHERE rownum <=2) WHERE sn=2;

//rownum是一次对数据进行标识，必须先有第一名再有第二名

SELECT * FROM emp WHERE rownum<=2;

SELECT empno,aname

    FROM(SELECT empno,ename FROM emp ORDER BY dbms_random.value())

WHERE  rownum <= 3;

SELECT *  FROM  v  WHERE  vname  LIKE ‘%CED%‘;

错误：SELECT * FROM  v  WHERE vname  LIKE   ‘_BCE%‘;

正确：SELECT * FROM v  WHERE  vname  LIKE  ‘\_BCE%‘   ESCAPE ‘\‘;

因为在LIKE语句中%和_都是通配符，我的理解就是个数不同而已

错误：SELECT   *  FROM     v    WHERE   vname  LIKE   ‘_\BCE%‘  ESCAPE ‘\‘ ;

正确：SELECT   *  FROM     v    WHERE   vname  LIKE   ‘_\\BCE%‘  ESCAPE ‘\‘ ;

结果：_\BCDF

解释：双写转义字符，百分号和下划线的处理方式一样

SELECT EMPNO,ENAME,HIREDATE   FROM emp WHERE deptno=10 ORDER BY 3 ASC;

SELECT empno,deptno,sal,ename,job FROM emp ORDER BY  2  ASC, 3  DESC;

SELECT TRANSLATE(‘ab 你好 bcadefg‘ , ‘abcdefg‘ , ‘1234567‘) AS   NEW_STR    FROM   DUAL;

SELECT data,translate(data,‘-0123456789‘,‘-‘) AS ename  FROM v ORDER BY 2 ;

思路：先将列中字母，也就是取出排序的依据另成一列ename，然后在排序，把数字和字母都替换为空；

select 信息

feom 表

where 时间列= last_day(时间列)-1

select 信息

from 表

where lower(last_name)=‘chen‘

select m.*

from employess e,employess m

where e.manager_id = m.manasger_id and e.last_name = ‘chen‘

select * 

from employess

where employess_id = (

                      select  manager_id  

                      from employess 

                      where last_name = "chen")

select employess_id , avg(salary)

from employess

group by employess_id

HAVING avg(SALARY)>8000

from employess

where employess_id = (

        select employess_id

        from employess

        group by employess_id

        having salary = min(avg(salary))

)