SQL IF SELECT query is null then do another query

Question

I have a query that regularly returns \"nothing\", and I would like to run a different query if this is the case, but I know not of the way of doing this. If anyone could be

Answer 1

There are some simple way only use sql.

Define your first query as a temp table, with union all, filter the second query with temp table's count.

with temp as (select * from t1 where 1=0)
select * from temp
union all
select * from t2 where (select count(*) from  temp) =0

This query will return the second table's records.

with temp as (select * from t1 )
select * from temp
union all
select * from t2 where (select count(*) from  temp) =0

And if temp query have result, only return temp query.

You can test with sql fiddle here.