Subquery sample

create table T3 (a int unique clustered, b int)

insert T1 values(0, 0, 0)
insert T1 values(1, 1, 1)

select
case
when T1.a > 0 then
(select T3.b from T3 where T3.a = T1.b)
else
T1.c
end
from T1

Reference: here