How many churches are there?
select count ( * ) from church
How many churches had weddings in 2015?
select count ( * ) from church as t1 join wedding as t2 on t1.church_id = t2.church_id where t2.year = 2015
Show all church names except or those that had a wedding in year 2015.
select name from church except select t1.name from church as t1 join wedding as t2 on t1.church_id = t2.church_id where t2.year = 2015