OCP-1Z0-新051-61题版本-2

QUESTION NO: 2

You need to design a student registration database that contains several tables storing academic information.

The STUDENTS table stores information about a student. The STUDENT_GRADES table stores information about the student's grades. Both of the tables have a column named STUDENT_ID.The STUDENT_ID column in the STUDENTS table is a primary key.

You need to create a foreign key on the STUDENT_ID column of the STUDENT_GRADES table that points to the STUDENT_ID column of the STUDENTS table. Which statement creates the foreign key?

A. CREATE TABLE student_grades (student_id NUMBER(12),semester_end DATE, gpa

NUMBER(4,3), CONSTRAINT student_id_fk REFERENCES (student_id) FOREIGN KEY

students(student_id));

B. CREATE TABLE student_grades(student_id NUMBER(12),semester_end DATE, gpa

NUMBER(4,3), student_id_fk FOREIGN KEY (student_id) REFERENCES students(student_id));

C. CREATE TABLE student_grades(student_id NUMBER(12),semester_end DATE, gpa

NUMBER(4,3), CONSTRAINT FOREIGN KEY (student_id) REFERENCES students(student_id));

D. CREATE TABLE student_grades(student_id NUMBER(12),semester_end DATE, gpa

NUMBER(4,3), CONSTRAINT student_id_fk FOREIGN KEY (student_id) REFERENCES

students(student_id));

Answer: D

答案解析：

参考：http://docs.oracle.com/cd/E11882_01/server.112/e41084/clauses002.htm#SQLRF52238

Foreign Key Constraint Example The following statement creates the

dept_20

table and defines and enables a foreign key on the

department_id

column that references the primary key on the

department_id

column
of the

departments

table:

CREATE TABLE dept_20
(employee_id NUMBER(4),
last_name VARCHAR2(10),
job_id VARCHAR2(9),
manager_id NUMBER(4),
hire_date DATE,
salary NUMBER(7,2),
commission_pct NUMBER(7,2),
department_id CONSTRAINT fk_deptno
REFERENCES departments(department_id) );

The constraint

fk_deptno

ensures that all departments given for employees in the

dept_20

table are present in the

departments

table. However, employees can have null department numbers, meaning
they are not assigned to any department. To ensure that all employees are assigned to a department, you could create a

NOT

NULL

constraint on the

department_id

column in the

dept_20

table in addition to the

REFERENCES

constraint.
Before you define and enable this constraint, you must define and enable a constraint that designates the

department_id

column of the

departments

table as a primary or unique key.
The foreign key constraint definition does not use the

FOREIGN

KEY

clause, because the constraint is defined inline. The data type of the

department_id

column is not needed, because Oracle
automatically assigns to this column the data type of the referenced key.
The constraint definition identifies both the parent table and the columns of the referenced key. Because the referenced key is the primary key of the parent table, the referenced key column names are optional.
Alternatively, you can define this foreign key constraint out of line:

CREATE TABLE dept_20
(employee_id NUMBER(4),
last_name VARCHAR2(10),
job_id VARCHAR2(9),
manager_id NUMBER(4),
hire_date DATE,
salary NUMBER(7,2),
commission_pct NUMBER(7,2),
department_id,
CONSTRAINT fk_deptno
FOREIGN KEY (department_id)
REFERENCES departments(department_id) );

The foreign key definitions in both variations of this statement omit the

ON

DELETE

clause, causing Oracle to prevent the deletion of a department if any employee works in that department.