集合演算（union, intersect, except) に対応

[lemonadern]

Summary

SQLにおける代表的な集合演算 （UNION, INTERSECT, EXCEPT）に対応しました

-- union
select
	a	as	a
from
	b
union
select
	c	as	c
from
	b
;
-- intersect
select
	a	as	a
from
	b
intersect
select
	c	as	c
from
	b
;
-- except
select
	a	as	a
from
	b
except
select
	c	as	c
from
	b
;