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?
(Orders表的结构如上,你已经插入了一些数据在表中,一段时间后,你想更新这个表,在表中把Ord_ID设置为主键约束,关于这个方案哪个语句是正确的?)
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.