ORACLE 优化查询改写二

--2.1 以指定的次序返回查询结果
      --查看所雇员工的信息
      SELECT empno,ename,hiredate FROM emp WHERE deptno=10 ORDER BY hiredate ASC;
      SELECT empno,hiredate FROM emp WHERE deptno=10 ORDER BY 3 ASC;
      SELECT empno,sal FROM emp WHERE deptno=10 ORDER BY 3 ASC;      
--2.2按多个字段排序
      --多列排序时，若前面的列有重复值，后面的排序才有用。
      SELECT empno,deptno,sal,job FROM emp ORDER BY 2 ASC,3 DESC;
--2.3按子串排序
      SELECT last_name AS 名称,phone_number AS 号码,salary AS 工资,substr(phone_number,-4) AS 尾号
      FROM hr.employees where rownum<=5 ORDER BY 4;
      
--2.4 TRANSLATE(expr,from_string,to_string)
      SELECT TRANSLATE('ab 你好 bcadefg','abcdefg','1234567') AS NEW_STR FROM DUAL;   
      --如果to_string为空，则返回空值   
      SELECT TRANSLATE('ab 你好 bcadefg','') AS NEW_STR FROM DUAL;
      --如果to_string对应的位置没有字符,删除from_string中列出的字符将会被取消
      SELECT TRANSLATE('ab 你好 bcadefg','1') AS NEW_STR FROM DUAL;--1 你好 1
--2.5按数字和字母混合字符串中的字母排序
      CREATE OR REPLACE VIEW V as SELECT empno || ' '|| ename AS data FROM emp;
      SELECT * FROM　V;
      SELECT data,TRANSLATE(data,' 0123456789','-0123456789') AS ename FROM V ORDER BY 2;
--2.6处理排序的空值
      --将空值排在最前面
      SELECT ename,comm FROM emp ORDER BY 3 NULLS FirsT;    
--2.7根据条件取不同列中的值比较
      SELECT empno AS 编码,ename AS 姓名,CASE 
               WHEN sal>=1000 AND sal<=2000 THEN 1 
               ELSE 2
             END AS 级别,sal AS 工资 from emp where deptno=30 ORDER BY 3,4;

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