Question

I have query I need to perform in MySQL on three linked tables. I can do it the lazy way with nested queries but I cannot work out how to do it with a single query.

Tables are:

Area:-
 : id     (int)
 : name   (string)

Consultant:-
 :id      (int)
 :active  (1/0)

ConsArea:-
 : areaID        (int)
 : consultantID  (int)

I need to loop through all the areas (with $area variable) so that I list ALL areas and against each area to indicate the number of ‘active’ consultants … so all areas must be listed with a value next to it (which can be zero if there are no active consultants associated)

The first part of the query (irrespective of consultant active or not) I can do with:

      SELECT areas.name AS aname, COUNT(consAreas.areaID) AS cct 
        FROM areas LEFT OUTER JOIN consAreas 
          ON consAreas.areaID = areas.id 
       WHERE areas.areaID = $area 
    GROUP BY areas.id 
    ORDER BY areas.name

.. but when I want to include the condition of the consultant being active I cannot work out the correct join. It only lists the areas with > 0 active consultants whereas I need all areas.

      SELECT areas.name AS aname, COUNT(consAreas.area) AS cct 
        FROM areas LEFT OUTER JOIN consAreas 
          ON consAreas.area = areas.id 
        **JOIN consultants ON consultants.id = consAreas.cons**
       WHERE areas.areaID = $area 
         **AND consultants.active = 1**
    GROUP BY areas.id 
    ORDER BY areas.name

Anyone help?

Answer 1

This is because of a behaviour of mysql. An inner join following a left join makes that left join an inner join.

SELECT areas.name AS aname, COUNT(consultants.id) AS cct 
FROM areas
    LEFT JOIN consAreas ON consAreas.area = areas.id 
    LEFT JOIN consultants ON consultants.id = consAreas.cons AND consultants.active = 1
WHERE 
    areas.areaID = $area 
GROUP BY areas.id 
ORDER BY areas.name

Here you can see I’m only using left joins, and more importantly filtering the consultants.active status from the left join ON clause directly.