Hierarchical Selection Within A Select Statment

CREATE TABLE RS_A(ColA char(1), ColB varchar(10))

INSERT INTO RS_A
VALUES ('S', 'shakespeare')
INSERT INTO RS_A
VALUES ('B', 'shakespeare')
INSERT INTO RS_A
VALUES ('P', 'shakespeare')

INSERT INTO RS_A
VALUES ('S', 'milton')
INSERT INTO RS_A
VALUES ('P', 'milton')
INSERT INTO RS_A
VALUES ('B', 'shelley')

INSERT INTO RS_A
VALUES ('B', 'kafka')
INSERT INTO RS_A
VALUES ('S', 'kafka')

INSERT INTO RS_A
VALUES ('P', 'tennyson')


SELECT * FROM RS_A

Now i need a select which selects based on hierarchy

if ColA = 'S', then select only that row
else if ColA = 'B' then select only that row
else if colA = 'P' then select only that row

So my results should look like
S shakespeare
S milton
B shelley
S kafka
P tennyson

Is there a way to do this within a select statement
I tried using a CASE in WHERE CLAUSE but it put out all rows which
existed/

If any of you can help me with this right away, its is greatly
appreciated
Thanks in advance