OCP 1Z0 051 15

15. View the Exhibit and examine the data in the CUSTOMERS table.  
Evaluate the following query: 
SQL>  SELECT  cust_name  AS  "NAME",  cust_credit_limit/2  AS  MIDPOINT,MIDPOINT+100  AS  "MAX 
LOWER LIMIT" 
FROM customers; 
The above query produces an error on execution. 
What is the reason for the error?  

A. An alias cannot be used in an expression. 
B. The a lias NAME should not be enclosed with in double quotation marks . 
C.  The  MIDPOINT+100  expression   gives  an  error  because   CUST_CREDIT_LIMIT  contains  NULL 
values. 
D.  The  a  lias  MIDPOINT  should  be  enclosed  with  in  double  quotation  marks   for  the 
CUST_CREDIT_LIMIT/2 expression . 

别名不能在同一范围内被另一列引用。
正确的写法应该是嵌套一层

SQL> show user;
User is "test"

SQL> 
SQL> CREATE TABLE customers(cust_no,cust_name,cust_city,cust_credit_limit) AS
  2  SELECT 101,'KING','NEW_YORK',100000 FROM DUAL UNION ALL
  3  SELECT 102,'GREEN','BOSTON',150000 FROM DUAL UNION ALL
  4  SELECT 103,'SCOTT','LONDON',NULL FROM DUAL UNION ALL
  5  SELECT 104,'SMITH','BOSTON',NULL FROM DUAL;
Table created

SQL> SELECT "NAME",MIDPOINT,MIDPOINT+100  AS  "MAX
  2  LOWER LIMIT"
  3  FROM
  4  (
  5  SELECT  cust_name  AS  "NAME",  cust_credit_limit/2  AS  MIDPOINT
  6  FROM customers);
NAME    MIDPOINT MAX
LOWER LIMIT
----- ---------- ---------------
KING       50000           50100
GREEN      75000           75100
SCOTT            
SMITH            

Answer: A