1Z0-051 QUESTION 31 关于check约束

31. Evaluate the following SQL commands: 
SQL>CREATE SEQUENCE ord_seq     
INCREMENT BY 10     
START WITH 120     
MAXVALUE 9999     
NOCYCLE; 
SQL>CREATE TABLE ord_items        
(ord_no NUMBER(4)DEFAULT ord_seq.NEXTVAL NOT NULL,         
item_no NUMBER(3),         

qty NUMBER(3) CHECK (qty BETWEEN 100 AND 200),     

expiry_date date CHECK (expiry_date > SYSDATE),         
CONSTRAINT it_pk PRIMARY KEY (ord_no,item_no),         
CONSTRAINT ord_fk FOREIGN KEY(ord_no) REFERENCES orders(ord_no)); 
The command to create a table fails. Identify the reason for the SQL statement failure? (Choose all that 
apply.) 
A. You cannot use SYSDATE in the condition of a CHECK constraint. 
B. You cannot use the BETWEEN clause in the condition of a CHECK constraint. 
C. You cannot use the NEXTVAL sequence value as a DEFAULT value for a column. 
D. You cannot use ORD_NO and ITEM_NO columns as a composite primary key because ORD_NO is 
also the FOREIGN KEY

答案：AC

解析：

ABD解析参看30题

http://blog.youkuaiyun.com/create_oracle/article/details/25559371

C选项，NEXTVAL不能在DEFAULT中使用，参看官方文档：

http://docs.oracle.com/cd/E11882_01/server.112/e41084/statements_7002.htm#SQLRF54458

Restriction on Default Column Values A DEFAULT expression cannot contain references to PL/SQL functions or to other columns, the pseudocolumns CURRVAL, NEXTVAL, LEVEL, PRIOR, and ROWNUM, or date constants that are not fully specified.