choose three
Examine this SQL statement:
SELECT custid, cus last_name "Last Name"
FROM customers
WHERE country_id = 10
UNION
SELECT cust_id CUST_NO, cust_last_name
FROM customers
WHERE country_id = 30
Identify three ORDER BY clauses, any one of which can complete the query successfully.
A) ORDERBY 2, 1
B) ORDER BY "CUST_NO"
C) ORDER BY 2,cust_id
D) ORDER BY CUST_NO
E) ORDER BY "Last Name"
Answer：ACE
(解析：此题的解析跟下面的解析一样，答案就是按照解析的总结规律找出来的。)
071-140 题目：
SELECT department_id "DEPT_ID", department_name, ‘b‘ FROM departments
WHERE department_id=90 UNION
SELECT department_id, department_name DEPT_NAME, ‘a‘ FROM departments
WHERE department_id=10
Which two ORDER BY clauses can be used to sort output?
A. ORDER BY DEPT_NAME;
B. ORDER BY DEPT_ID;
C. ORDER BY ‘b‘;
D. ORDER BY 3;
Correct Answer: BD
(解析：通过实验发现别名必须是用双引号概括起来的才可以用来排序;而且发现取了别名(别名用双引号概括)的列的名字也不能用来排序;没有用双引号概括起来的别名的列名可以用来排序;order by 3，可以根据显示的列的顺序进行排序。
注意 union 排序的的时候 order by 要写在语句的最后;发现只能在第一条 select 语句中的列进行排序。)
SQL> select empno "CUST_NO",ename "LAST NAME",‘a‘ FROM emp
2 UNION
3 SELECT EMPNO ,ENAME,‘b‘ FROM emp
4 ORDER BY empno;
ORDER BY empno
*
第 4 行出现错误:
ORA-00904: "EMPNO": 标识符无效
原文地址：https://blog.51cto.com/13854012/2451432