OCP-1Z0-051-2015-36题

Which CREATE TABLE statement is valid? 找出创建表的格式正确的语句

A.

CREATE TABLE ord_details

(ord_no NUMBER(2) PRIMARY KEY,

item_no NUMBER(3) PRIMARY KEY,

ord_date DATE NOT NULL);

B.

CREATE TABLE ord_details

(ord_no NUMBER(2) UNIQUE, NOT NULL,

item_no NUMBER(3),

ord_date DATE DEFAULT SYSDATE NOT NULL);

C.

CREATE TABLE ord_details

(ord_no NUMBER(2) ,

item_no NUMBER(3),

ord_date DATE DEFAULT NOT NULL,

CONSTRAINT ord_uq UNIQUE (ord_no),

CONSTRAINT ord_pk PRIMARY KEY (ord_no));

D.

CREATE TABLE ord_details

(ord_no NUMBER(2),

item_no NUMBER(3),

ord_date DATE DEFAULT SYSDATE NOT NULL,

CONSTRAINT ord_pk PRIMARY KEY (ord_no, item_no));

Answer: D
Explanation:

A PRIMARY KEY constraint creates a primary key for the table. Only one primary key can be created for each table. 

一个表只有一个主键。A错。UNIQUE, NOT NULL之间不要逗号，列级约束不要逗号，B错

一个DEFAULT 后面没有指定默认值，C错

The PRIMARY KEY constraint is a column or a set of columns that uniquely identifies each row in a table. 

This constraint enforces the uniqueness of the column or column combination and ensures that no column that is part of the primary key can contain a null  value.

Note: Because uniqueness is part of the primary key constraint definition, 

the Oracle server enforces the uniqueness by implicitly （隐含的唯一性）creating a unique index on the primary key column or columns.