在SQL中：当“无结果”从查询返回时显示一些默认结果

Question

I'm using Postgres to get results from a table. I want to use SQL to solve a problem.

When my postgresql query returns no results, I would like some default row results to appear. I am using Jasper-reports, so I don't have a lot of tools to catch the event where there are no results, and then deliver default results.

When the first query DOES show results I do not want the default results to appear. Currently I'm trying to use a UNION but I want the second query after the Union command to ONLY happen if the first query shows 0 results.

Can this be done in SQL?

Answer 1

You could try a CTE:

with fooresults as
(
select c1, c2 from foo
)

select c1, c2 from fooresults
union
select c1, c2 from blah
where (select count(*) from fooresults)=0

OR you can test for empty fooresults without counting:

 where not exists (select 1 from fooresults) 

PS And of course you could add any conditions required when you instantiate the CTE:

      select c1, c2 from foo where .... <some conditions>

Answer 2

Don't do a count to check the existence of a row as it will count the whole table. In instead use exists

select c1, c2, c3
from t
union
select 1, 2, 3
where not exists (select * from t)

Answer 3

SELECT * 
FROM table1
UNION ALL
SELECT *
FROM table2
WHERE 0 = (
   SELECT count(table1.id)
   FROM Table1
   GROUP BY table1.id)

By head, feels ugly though...