PostgreSQL - Consultas complejas

SELECT forum_id, forum_project, forum_description, forum_owner, forum_name,forum_moderated, forum_create_date,
forum_last_date,sum(case c.message_parent=-1 when true then 1 when false then 0 ELSE null END) as forum_topics,
sum(case c.message_parent >0 when true then 1 when false then 0 ELSE null END) as forum_replies,
user_username, project_name, project_color_identifier, (contact_first_name ||','||contact_last_name) AS owner_name,
SUBSTRING(l.message_body from 1 for 30) AS message_body,LENGTH(l.message_body) AS message_length, watch_user, l.message_parent,
l.message_id,count(distinct v.visit_message) as visit_count, count(distinct c.message_id) as message_count
FROM "projects" AS pr,"users" AS u,"forums" AS forums
LEFT JOIN "forum_messages" AS l ON l.message_id = forum_last_id
LEFT JOIN "forum_messages" AS c ON c.message_forum = forum_id
LEFT JOIN "forum_watch" AS w ON watch_user = 1 AND watch_forum = forum_id
LEFT JOIN "forum_visits" AS v ON visit_user = 1 AND visit_forum = forum_id and visit_message = c.message_id
LEFT JOIN "contacts" AS cts ON contact_id = u.user_contact
LEFT JOIN "companies" AS co ON co.company_id = pr.project_company
LEFT JOIN "project_departments" AS project_departments ON pr.project_id = project_departments.project_id
LEFT JOIN "departments" AS dep ON dep.dept_id=project_departments.department_id
WHERE user_id = forum_owner AND pr.project_id = forum_project AND project_active = 1
GROUP BY forum_id,forum_project,forum_description,forum_owner, forum_name,forum_moderated, forum_create_date, user_username, project_name, project_color_identifier,
forum_last_date
ORDER BY forum_name asc

Al correrlo me da el siguiente error:

ERROR: invalid reference to FROM-clause entry for table "u"
HINT: There is an entry for table "u", but it cannot be referenced from this part of the query.

Cómo soluciono este problema???

"Este error es por usar unos 'joins' definidos con comas y otros con la sentencia JOIN en la misma consulta.

Segun el estandar SQL la palabra JOIN tiene mayor prioridad que la coma cuando se definen joins y se evalua internamente la consulta. Por ello, el alias 'u' que defines con '"users" AS u' no esta disponible en el 'ON contact_id = u.user_contact'.

No definas joins de dos maneras diferentes en la misma consulta. Define '"projects" AS pr,"users" AS u,"forums" AS forums' con sentencias JOIN.

--
Rafael Martinez"