Unknown column 'b.Hostname' in 'on clause' - mysql

Question

I have the following MYSQL Query - trying to join 3 tables and find unique information(data)
SELECT a.LocationID, a.Model, a.SerialNum,a.Purpose, b.IP, a.Services,a.DeviceID, COUNT(a.Hostname)
FROM RefConnection.Equipment_Info a, RefConnection.Connections b, RefConnection.VM_Info c
JOIN Equipment_Info on b.Hostname = a.Hostname
WHERE a.Hostname = c.Hostname
AND b.status = a.Status
AND a.status = c.Status
GROUP BY a.LocationID, a.Model, a.SerialNum, a.Purpose, b.IP, a.Services, a.DeviceID
ORDER BY COUNT(b.Hostname)DESC;
This works with 2 tables :
SELECT d.locationID, d.Model, d.SerialNum, d.Status, da.IP, COUNT(d.HOSTNAME)
FROM RefConnection.Equipment_Info d, RefConnection.Connections da
WHERE d.Hostname = da.Hostname
AND d.Status = da.Status
Group By d.locationID, d.Model, d.locationID, d.Model, d.SerialNum, d.Status, da.IP
ORDER BY COUNT(da.Hostname) DESC;
Unknown column 'b.Hostname' in 'on clause'

Answer 1

Never use commas in the FROM clause. Always use proper, explicit, standard JOIN syntax.
FROM RefConnection.Equipment_Info a JOIN
RefConnection.Connections b
ON b.Hostname = a.Hostname AND
b.status = a.Status JOIN
RefConnection.VM_Info c
ON a.Hostname = c.Hostname AND
a.status = c.Status
Your problem is the scoping rules for identifiers. This does not work as you expect with commas -- yet another reason to avoid them.
I don't know what Equipment_Info is doing a second time in the query.
I would also strongly advise you to use meaningful table aliases, such as e, c, and i rather than arbitrary characters.