How to update data in table?

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SQL-Server

SQL-Server-2008R2

, +

I have following tables

create table emp
(
	empid int primary key identity(1,1),
	name varchar(50)
)

create table rolemaster
(
	roleid int primary key identity(1,1),
	[role] varchar(50)
)

create table map
(
	empid int foreign key references emp(empid),
	roleid int foreign key references rolemaster(roleid),
)

create table sal
(
	empid int foreign key references emp(empid),
	salary float
)

I want to update salary of employee whose roleid is 1 by 20% and update salary of employee whose roleid is 2 by 30% in single update statement. Plz help me.

What I have tried:

update sal set salary=case empid
when (select empid from map where roleid=(select roleid from rolemaster where role='a')) then salary*0.2
when (select empid from map where roleid=(select roleid from rolemaster where role='c')) then salary*0.5
end

but did not work.