ocp試題解析：11g-1Z0-051-v9.02-034題
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34. You created an ORDERS table with the following description:
name Null Type
ORD_ID NOT NULL NUMBER(2)
CUST_ID NOT NULL NUMBER(3)
ORD_DATE NOT NULL DATE
ORD_AMOUNT NOT NULL NUMBER (10,2)
You inserted some rows in the table. After some time, you want to alter the table by creating the
PRIMARY KEY constraint on the ORD_ID column. Which statement is true in this scenario?
A. You cannot have two constraints on one column.
B. You cannot add a primary key constraint if data exists in the column.
C. The primary key constraint can be created only at the time of table creation .
D. You can add the primary key constraint even if data exists, provided that there are no duplicate values.
Answer: D
試題解析：
A. 一個列上不能有兩個約束,錯誤，例如非空+唯一可以作用在一個列上。
B. 如果數據已經存在不能添加主鍵約束，錯誤，有數據存在，也可以插入主鍵約束，前提是數據沒有重復
C. 主鍵約束只有在表創建時才能被創建，錯誤。有了數據之后一樣可以增加，不一定只有在創建的時候加
D. 即使數據存在，如果沒有重復值，你能添加主鍵約束，正確。
