SQL & PL/SQL :: How To Create Random Shift Rota

I wanted to create shift rota for the employees as per the below conditions.

1). Shift should be rotated.(it should check previous 2 shifts and assign to another shift).

2). The same employees should not be allocated for the next shift .

Ex : Now jones, jekku, henry and villiams are in "C" shift . So the same employees should not be allocated to the next shift, it should be random. Some of these employees should be allocated to "B" and some employees should allocated to "A" shift....

Below are the SQL statements to create the table and input the data..

create table resource_details
(empno number(5) ,
ename varchar2(30),
shift char(1));

alter table resource_details add constraint res_pk primary key(empno) ;

insert into resource_details values(51370 , 'Finney' , 'A') ;
insert into resource_details values(51371, 'Jerry' , 'B') ;
insert into resource_details values(51372, 'Jones' , 'C') ;
insert into resource_details values(51373, 'Smith' , 'A') ;
insert into resource_details values(51374, 'Krishna' , 'B') ;
insert into resource_details values(51375,'Chenna','A');

[code]....

I have tried with employee number to check which shift he is/was in & assigned the shift accordingly , but here shift is getting rotated but not employees(same employees are going to the next shift, which should not be accepted..)