Question

I have three tables

tbl_worker

City   WorkerName     Job
 10     Raymond    Carpenter
 10     Sara       Programmer 
 11     John       Painter
 11     Alice      Booker

tbl_workshop

City    WorkshopName   
 10       Dell         
 11       Apple
 10       Sony 

Tbl_city
  Id    Name
  10    Mexico
  11    Sydney

Now I want such a result:

City    WorkersQty    WorkshopQry
 Mexico     2               2
 Sydney     2               1

The result returns the quantity of workers and workshops which are registered in different cities.
what query should I write?

Answer 1

Group the tbl_workshop and tbl_worker tables individually, then use a LEFT JOIN to get the results:

SELECT c.City,
    k.Cnt AS WorkersQty,
    w.Cnt AS WorkshopQry
FROM Tbl_city c
  LEFT JOIN (
    SELECT City, COUNT(*) Cnt
    FROM tbl_worker
    GROUP BY City
    ) k ON c.ID = k.City
  LEFT JOIN (
    SELECT City, COUNT(*) Cnt
    FROM tbl_workshop
    GROUP BY City
    ) w ON c.ID = w.City

If you want to only show the cities that have workshop’s or workers, add:

WHERE k.Cnt > 0 OR w.Cnt > 0