В этом случае, похоже, что самый простой способ сделать это - объединить наборы результатов - например, так:
select reviews.id_user,
reviews.created_time,
reviews.id review_id,
reviews.review,
null placespop_id,
null id_pop,
null usersfoursquarecheckin_id,
null usersfacebookcheckins_id,
null placesphotos_id,
null usersbadges_id
from reviews
where reviews.id_user in (1,2,3,4,5)
union all
select placespops.id_user,
placespops.created_time,
null review_id,
null review,
placespops.id placespop_id,
placespops.id_pop,
null usersfoursquarecheckin_id,
null usersfacebookcheckins_id,
null placesphotos_id,
null usersbadges_id
from placespops
where placespops.id_user in (1,2,3,4,5)
union all
select usersfoursquarecheckins.id_user,
usersfoursquarecheckins.created_time,
null review_id,
null review,
null placespop_id,
null id_pop,
usersfoursquarecheckins.id usersfoursquarecheckin_id,
null usersfacebookcheckins_id,
null placesphotos_id,
null usersbadges_id
from usersfoursquarecheckins
where usersfoursquarecheckins.id_user in (1,2,3,4,5)
union all
select usersfacebookcheckins.id_user,
usersfacebookcheckins.created_time,
null review_id,
null review,
null placespop_id,
null id_pop,
null usersfoursquarecheckin_id,
usersfacebookcheckins.id usersfacebookcheckins_id,
null placesphotos_id,
null usersbadges_id
from usersfacebookcheckins
where usersfacebookcheckins.id_user in (1,2,3,4,5)
union all
select placesphotos.id_user,
placesphotos.created_time,
null review_id,
null review,
null placespop_id,
null id_pop,
null usersfoursquarecheckin_id,
null usersfacebookcheckins_id,
placesphotos.id placesphotos_id,
null usersbadges_id
from placesphotos
where placesphotos.id_user in (1,2,3,4,5)
union all
select usersbadges.id_user,
usersbadges.created_time,
null review_id,
null review,
null placespop_id,
null id_pop,
null usersfoursquarecheckin_id,
null usersfacebookcheckins_id,
null placesphotos_id,
usersbadges.id usersbadges_id
from usersbadges
where usersbadges.id_user in (1,2,3,4,5)
order by 2 desc