Which churches had a wedding in year 2015?
select * from church as t1 join wedding as t2 on t1.church_id = t2.church_id where t2.year = 2015
Show all names of the churches except for those.
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