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.