How to use IF-THEN-ELSE statement for a sub-query of my main select statement

Hi Gurus

I have an interesting problem that I am battling to resolve. I have written a sql query that spans multiple sub-queries in the from section of the main query. i.e.

Select ax.a,

by.b

from table z

(select 1 a,

2 b

from table x) ax,

(select 2 b

2 c

from table y) by

where ax.b = z.b

and by.c = z.b

and z.assignment_id = 180

Now if each sub-query returns rows the query will return data, however let us say the first sub has not data for assignment 180 but the second sub-query does have data for assignment 180 then the complete query will return no rows.