Case? Decode?

Hello,

I need to do the following:

Lets say we have the following table.

A B C
1 XYZ XYZ
100

Now if the value of a is "100" then i need to do a join and get a value from another table, else I put the value of b a "test"

I am trying to do this using case

like this

select case
(when a = 100 then (select col from ....where newtable.id = a.id
else "test"
end) as xyz
from a,b,other tables.

But the problem is that the subquery returns more than one row..(it should return one row)

How do I do this?