CREATE FUNCTION dbo.udf_GetColonistsCount(@planetName VARCHAR(30))
RETURNS TABLE
AS

RETURN
(SELECT
    p.Name AS [PlanetName],
    COUNT(tc.ColonistId) AS [Count]
FROM Planets AS p
JOIN Spaceports AS sp ON sp.PlanetId = p.Id
JOIN Journeys AS j ON j.DestinationSpaceportId = sp.Id
JOIN TravelCards AS tc ON tc.JourneyId = j.Id
WHERE p.Name = @planetName
GROUP BY p.Name)