SQL是/否查询，没有控制流功能（IF，CASE，COALESCE等）

Question

I've been trying to find a way to get a Yes/No answer to a query without using control flow functions(IIF,ELSE,CASE,COALESCE,ISNULL,IFNULL,etc). I want my query to give me a "Yes" answer if there has been a flight of a certain Airline on a given date between 2 given airports. This is what i have done so far

SELECT 'Yes' as Answer
FROM flights
WHERE flights.date = '2014-12-12' AND flights.routes_id IN (SELECT routes.id
                                                            FROM routes
                                                            INNER JOIN airlines
                                                            ON airlines.id = routes.airlines_id
                                                            WHERE airlines_id IN (SELECT airlines.id
                                                                                 FROM airlines
                                                                                 WHERE airlines.name='Olympic Airways')
                                                            AND routes.source_id = (SELECT airports.id
                                                                                    FROM airports
                                                                                    WHERE airports.name like '%Venizelos%')
                                                            AND routes.destination_id=(SELECT airports.id
                                                                                    FROM airports
                                                                                    WHERE airports.name like 'London Gatwick')) 
UNION
SELECT 'No' AS Answer
FROM flights
WHERE flights.date = '2014-12-12' AND flights.routes_id  NOT IN (SELECT routes.id
                                                                FROM routes
                                                                INNER JOIN airlines
                                                                ON airlines.id = routes.airlines_id
                                                                WHERE airlines_id IN (SELECT airlines.id
                                                                                    FROM airlines
                                                                                    WHERE airlines.name = 'Olympic Airways')
                                                                AND routes.source_id = (SELECT airports.id
                                                                                    FROM airports
                                                                                    WHERE airports.name like '%Venizelos%')
                                                                AND routes.destination_id=(SELECT airports.id
                                                                                    FROM airports
                                                                                    WHERE airports.name like 'London Gatwick'))

The problem is that when i run the queries it prints both Yes and No, when it should print only one of those two. How can it be fixed?

Answer 1

This happens because in that day there will be flight for that airline, but also for others, from/to that airports but also from others.

Put your query inside another query like:

SELECT Max(Answer)
from (
      your query from the question
)

This will return Yes if you have at leas a good flight (because the internal query will return Yes and No ) and No if you have only no matches (the results from the internal query will be only No ).

Hope this helps.