如下有三道面试题：
创建两张表格：
CREATE TABLE CUSTOMERS(ID NUMBER(2),NAME VARCHAR2(10));CREATE TABLE ORDERS(ID NUMBER(2),CUSTOMERID NUMBER(2));SELECT * FROM CUSTOMERS;INSERT INTO CUSTOMERS VALUES(1,'JOE');INSERT INTO CUSTOMERS VALUES(2,'HENRY');INSERT INTO CUSTOMERS VALUES(3,'SAM');INSERT INTO CUSTOMERS VALUES(4,'MAX');SELECT * FROM ORDERS;INSERT INTO ORDERS VALUES(1,3);INSERT INTO ORDERS VALUES(2,1);解题答案：
/* 某网站包含两张表，Customer和Orders表。编写一个SQL查询。找出所有从不订购任何东西的用户。*/SELECT * FROM CUSTOMERS WHERE ID NOT IN(SELECT CUSTOMERID FROM ORDERS)；
创建两张表格：
CREATE TABLE T_EMAIL(ID NUMBER(2) PRIMARY KEY,EMAIL VARCHAR2(20));SELECT * FROM T_EMAIL;INSERT INTO T_EMAIL VALUES(1,'ganzexin@163.com');INSERT INTO T_EMAIL VALUES(2,'ganzexin@163.com');INSERT INTO T_EMAIL VALUES(3,'1395520340@qq.com');解题答案：
-- 删除重复项DELETE FROM T_EMAIL WHERE ROWID NOT IN(SELECT MIN(ROWID) FROM T_EMAIL GROUP BY EMAIL -- 记得要写Group by 要不然就只会) DELETE FROM T_EMAIL WHERE EMAILIN (SELECT EMAIL FROM T_EMAIL GROUP BY EMAIL HAVING COUNT(*)>1) And ROWID Not In (Select Min(ROWID) From T_EMAIL Group By EMAIL Having Count(*) > 1);删除重复值：ROWID
注意事项：
1.ROWID的使用——快速删除重复的记录
2.rowid去重（删除表的重复记录）
创建两张表格：
CREATE TABLE T_EMPLOYEE(ID NUMBER(3) PRIMARY KEY,NAME VARCHAR2(10),SALARY NUMBER(7,2),DEPTNO NUMBER(2));CREATE TABLE T_DEPTARTMENT(ID NUMBER(2) PRIMARY KEY,NAME VARCHAR2(10));INSERT INTO T_EMPLOYEE VALUES(1,'JOE',7000,1);INSERT INTO T_EMPLOYEE VALUES(2,'HENRY',8000,2);INSERT INTO T_EMPLOYEE VALUES(3,'SAM',7000,2);INSERT INTO T_EMPLOYEE VALUES(4,'MAX',10000,1);INSERT INTO T_DEPTARTMENT VALUES(1,'IT');INSERT INTO T_DEPTARTMENT VALUES(2,'SALES');SELECT * FROM T_EMPLOYEE;SELECT * FROM T_DEPTARTMENT;解题答案：
--找出每个部门工资最高的员工SELECT * FROM (SELECT TDEPT.NAME AS DEPTNAME,TEMP.NAME,TEMP.SALARY,ROW_NUMBER()OVER(PARTITION BY TDEPT.NAME ORDER BY TEMP.SALARY DESC)AS RK FROM T_EMPLOYEE TEMPJOIN T_DEPTARTMENT TDEPTON TEMP.DEPTNO =TDEPT.ID)WHERE RK =1;