单项选择题

Which statement explicitly names a constraint? （）

A. ALTER TABLE student_grades ADD FOREIGN KEY (student_id) REFERENCES students (student_id);
B. ALTER TABLE student_grades ADD CONSTRAINT NAME = student_id_fk FOREIGN KEY (student_id) REFERENCES students (student_id);
C. ALTER TABLE student_grades ADD CONSTRAINT student_id_fk FOREIGN KEY (student_id) REFERENCES students (student_id);
D. ALTER TABLE student grades ADD NAMED CONSTRAINT student_id_fk FOREIGN KEY (student_id) REFERENCES students (student_id);
E. ALTER TABLE student grades ADD NAME student_id_fk FOREIGN KEY (student_id) REFERENCES students (student_id);

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单项选择题

You want to display the titles of books that meet these criteria:
1. Purchased before January 21, 2001
2. Price is less then $500 or greater than $900
You want to sort the results by their data of purchase, starting with the most recently bought book. Which statement should you use? （）

A. SELECT book_title FROM books WHERE price between 500 and 900 AND purchase_date < '21-JAN-2001' ORDER BY purchase_date;
B. SELECT book_title FROM books WHERE price IN (500,900) AND purchase_date < '21-JAN-2001' ORDER BY purchase date ASC;
C. SELECT book_title FROM books WHERE price < 500 or > 900 AND purchase_date < '21-JAN-2001' ORDER BY purchase date DESC;
D. SELECT book_title FROM books WHERE (price < 500 OR price > 900) AND purchase_date < '21-JAN-2001' ORDER BY purchase date DESC;

单项选择题

Which /SQL*Plus feature can be used to replace values in the WHERE clause?（）

A. Substitution variables
B. Replacement variables
C. Prompt variables
D. Instead-of variables
E. This feature cannot be implemented through /SQL*Plus.

单项选择题

Examine the description of the MARKS table:
STD_ID NUMBER(4)
STUDENT_NAME VARCHAR2(30)
SUBJ1 NUMBER(3)
SUBJ2 NUMBER(3)
SUBJ1 and SUBJ2 indicate the marks obtained by a student in two subjects. Examine this SELECT statement based on the MARKS table:
SELECT subj1+subj2 total_marks, std_id
FROM marks
WHERE subj1 > AVG(subj1) AND subj2 > AVG(subj2) ORDER BY total_ marks;
What is the result of the SELECT statement? （）

A. The statement executes successfully and returns the student ID and sum of all marks for each student who obtained more than the average mark in each subject.
B. The statement returns an error at the SELECT clause.
C. The statement returns an error at the WHERE clause.
D. The statement returns an error at the ORDER BY clause.