SQL & PL/SQL

SQL> desc grade;
Name Null? Type
----------------------------------------- -------- -------------------
SLNO CHAR(3)
IID NUMBER(5)
ID VARCHAR2(7)
SCORE NUMBER(6,3)
GRADE CHAR(1)

SQL> select* from grade;

SLN IID ID SCORE G
--- ---------- ------- ---------- -
000 10001 CS00000 86.333 B
001 10002 CS00001 86.333 B
001 10002 CS00002 86.666 B
001 10002 CS00003 85.466 B
002 10003 CS00004 79.227 C
002 10003 CS00005 79.545 C
003 10004 CS00006 59.4 F
003 10004 CS00007 93.6 A
004 10005 CS00008 83.6 B
004 10005 CS00009 94.6 A
005 10006 CS00010 90.266 A

SLN IID ID SCORE G
--- ---------- ------- ---------- -

2) SQL> desc showgrade;
Name Null? Type
----------------------------------------- -------- -----------------
SFNAME NOT NULL VARCHAR2(20)
SLNAME NOT NULL VARCHAR2(20)
CNO NOT NULL CHAR(6)
CSNO NOT NULL NUMBER(1)
SCORE NUMBER

SQL> select * from showgrade;

SFNAME SLNAME CNO CSNO SCORE
-------------------- -------------------- ------ ---------- ----------
Al Jamerncy CS 142 1 83.1
Deborah Reyes CS 140 1 83.1
E.A. Torres CS 142 1 83.375
Gary Aung CS 147 1 86.2666667
Gerard Biers CS 25 1 90.2666667
Gerry Tripp CS 132 1 87.1
Janet Jung CS 120 2 77.6333333
Jenny Golsmith CS 100 1 87.1
John Smith CS 124 1 83.1
Judy Sethi CS 25 2 85.3333333
Kathleen Mulroy CS 135 1 82.1

SFNAME SLNAME CNO CSNO SCORE
-------------------- -------------------- ------ ---------- ----------

I need to convert the scores obtained from 2nd table to letter grade based on
if score>90 then grade=A
if score>80 and <90 then B
if score>70 and <80 then C
if score>60 and <70 then D
else F

Plz let me know