Question

My database has following structure:

Every Region (CountryStates) can have many cities (Areas). Every Area can have many tourist attractions or points of interest (POIs). I want to get list of how many Areas and Attractions are there in each Region. If there is none, I want to display 0.

This is my query:

SELECT Tab1.Reg AS Reg, CountAreas, CountPois
FROM
(SELECT 
        c.Name AS Reg, 
        COUNT(a.Id) AS CountAreas
FROM  
       CountryStates as c LEFT JOIN
       Areas AS a ON a.CountryStates_Id = c.Id
GROUP BY c.Name 
) as Tab1 left join
(SELECT 
        c1.Name AS Reg, 
        COUNT(p.Id) AS CountPois
FROM  
       CountryStates as c1 LEFT JOIN
       Areas AS a ON a.CountryStates_Id = c1.Id LEFT JOIN
       POIs AS p ON a.Id = p.Areas_Id
GROUP BY c1.Name 

) as Tab2 on Tab1.Reg = Tab2.Reg

How can I make this query in just one SELECT?

Answer 1

This query returns the same result:

SELECT c.Name AS Reg, 
        COUNT(DISTINCT(a.Id)) AS CountAreas,
        COUNT(p.Id) AS CountPois
FROM CountryStates as c 
    LEFT JOIN Areas as a ON a.CountryStates_Id = c.Id
    LEFT JOIN POIs AS p ON a.Id = p.Areas_Id
GROUP BY c.Name