SQL with dynamic exclusion, condensed into one query (SQLite)

I have two tables: Table1: id1, field1, field2, field3 Table2: id2, field1, field3

What I need is:

"select id2 from table2 where field1 = x and field3 = y;"

if it r开发者_如何学运维eturns empty then perform and process the results from:

"select field2 from table1 where field1 = x and field3 = y;"

Is there a way to perform this in a single query in SQLite?

Assuming that the tables are joinable on field1 and field3, then something like this should work (Warning: Untested):

select
  case when sum(count_id2)>0 then a.id2
  else b.field2 end as column
from(
  select id2,count(1) as count_id2
  from table2
  where field1=x and field3=y
  group by id2
)a
join(
  select id2,field2
  from table1
  join table2
  using(field1,field3)
  where field1=x and field3=y
)b
using(id2);