Write a PL/SQL function that returns the average retail price of all of the books in the books table.

BOOKS table

CREATE TABLE Books

(ISBN VARCHAR2(10),

Title VARCHAR2(30),

PubDate DATE,

PubID NUMBER (2),

Cost NUMBER (5,2),

Retail NUMBER (5,2),

Discount NUMBER (4,2),

Category VARCHAR2(12),

CONSTRAINT books_isbn_pk PRIMARY KEY(isbn),

CONSTRAINT books_pubid_fk FOREIGN KEY (pubid)

REFERENCES publisher (pubid));

INSERT INTO BOOKS

VALUES ('1059831198','BODYBUILD IN 10 MINUTES A DAY','21-JAN-05',4,18.75,30.95, NULL, 'FITNESS');

INSERT INTO BOOKS

VALUES ('0401140733','REVENGE OF MICKEY','14-DEC-05',1,14.20,22.00, NULL, 'FAMILY LIFE');

INSERT INTO BOOKS

VALUES ('4981341710','BUILDING A CAR WITH TOOTHPICKS','18-MAR-06',2,37.80,59.95, 3.00, 'CHILDREN');

INSERT INTO BOOKS

VALUES ('8843172113','DATABASE IMPLEMENTATION','04-JUN-03',3,31.40,55.95, NULL, 'COMPUTER');

INSERT INTO BOOKS

VALUES ('3437212490','COOKING WITH MUSHROOMS','28-FEB-04',4,12.50,19.95, NULL, 'COOKING');

INSERT INTO BOOKS

VALUES ('3957136468','HOLY GRAIL OF ORACLE','31-DEC-05',3,47.25,75.95, 3.80, 'COMPUTER');

INSERT INTO BOOKS

VALUES ('1915762492','HANDCRANKED COMPUTERS','21-JAN-05',3,21.80,25.00, NULL, 'COMPUTER');

INSERT INTO BOOKS

VALUES ('9959789321','E-BUSINESS THE EASY WAY','01-MAR-06',2,37.90,54.50, NULL, 'COMPUTER');

INSERT INTO BOOKS

VALUES ('2491748320','PAINLESS CHILD-REARING','17-JUL-04',5,48.00,89.95, 4.50, 'FAMILY LIFE');

INSERT INTO BOOKS

VALUES ('0299282519','THE WOK WAY TO COOK','11-SEP-04',4,19.00,28.75, NULL, 'COOKING');

INSERT INTO BOOKS

VALUES ('8117949391','BIG BEAR AND LITTLE DOVE','08-NOV-05',5,5.32,8.95, NULL, 'CHILDREN');

INSERT INTO BOOKS

VALUES ('0132149871','HOW TO GET FASTER PIZZA','11-NOV-06',4,17.85,29.95, 1.50, 'SELF HELP');

INSERT INTO BOOKS

VALUES ('9247381001','HOW TO MANAGE THE MANAGER','09-MAY-03',1,15.40,31.95, NULL, 'BUSINESS');

INSERT INTO BOOKS

VALUES ('2147428890','SHORTEST POEMS','01-MAY-05',5,21.85,39.95, NULL, 'LITERATURE');