30. Evaluate the following CREATE TABLE commands:

CREATE TABLE orders

(ord_no NUMBER(2) CONSTRAINT ord_pk PRIMARY KEY,

ord_date DATE,

cust_id NUMBER(4)); 

CREATE TABLE ord_items

(ord_no NUMBER(2),

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 above command fails when executed. What could be the reason?

A. SYSDATE cannot be used with the CHECK constraint. 

B. The BETWEEN clause cannot be used for the CHECK constraint. 

C. The CHECK constraint cannot be placed on columns having the DATE data type. 

D. ORD_NO and ITEM_NO cannot be used as a composite primary key because ORD_NO is also the FOREIGN KEY. 





Answer: A

题目问，上面建表语句运行失败的原因？

A：sysdate不能用于check约束中（正确）

SQL> create table t

  2  ( x date check (x >sysdate));

( x date check (x >sysdate))

                   *

ERROR at line 2:

ORA-02436: date or system variable wrongly specified in CHECK constraint

B：Between子句不能用于check约束中（错误）

SQL> create table t (x int check (x between 10 and 100));

Table created.

C：check约束不能放置在含有Date数据类型的字段 （错误）

SQL> create table t ( x date check ( x > to_date('2014-01-12','YYYY-MM-DD')));

Table created.

D：ORD_NO and ITEM_NO不能被用作复合主键。由于ORD_NO同一时候也是外检（错误，一个字段能够同一时候被定义为主键和外检）

SQL>create table ord_items

  2  (ord_no number not null,

  3  item_no number,

  4  constraint ord_fk foreign key(ord_no) references orders(ord_no),

  5  constraint it_pk primary key(ord_no,item_no));

Table created.