if @a is not null
begin
--得到@a想得到的
end
if @b is not null
begin
--得到@b想得到的
end
if @c is not null
begin
--得到@c想得到的
end

if @c is not null
begin
delete c from @tb c where c.id not in (select id from tb where f = @c)
end
select * from @tb