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 .

[email protected]> SELECT cust_name AS "NAME", cust_credit_limit/2 AS MIDPOINT,MIDPOINT+100 AS "MAX 
  2  LOWER LIMIT" 
  3  FROM customers; 
SELECT cust_name AS "NAME", cust_credit_limit/2 AS MIDPOINT,MIDPOINT+100 AS "MAX
                                                            *
ERROR at line 1:
ORA-00904: "MIDPOINT": invalid identifier
 

報錯，select中不應該用別名來用在表示式裡

[email protected]> SELECT cust_last_name AS "NAME", cust_credit_limit/2 AS MIDPOINT,cust_credit_limit/2+100 AS "MAX LOWER LIMIT"
  2  FROM customers WHERE ROWNUM<6;

NAME                                                 MIDPOINT MAX LOWER LIMIT
-------------------------------------------------- ---------- ---------------
Ruddy                                                     750             850
Ruddy                                                    3500            3600
Ruddy                                                    5500            5600
Ruddy                                                     750             850
Ruddy                                                    4500            4600

此題答案選A