1Z0-051 QUESTION 34 通过alter table修改primary key

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. 

答案为：D

A选项错误，orders表本来没有primary key，所以可以创建primary约束；

B选项错误，如果数据是没有重复的，唯一的，可以创建primary key；

C选项错误，可以通过alter table orders add constraint pk_orders primary key(ird_id);修改表来创建primary key；

D选项正确，存在数据而且没有重复的数据，可以创建primary key。