集合运算

union、intersect、except对应于∪、∩、－运算

并运算union

union自动去除重复，如果想保留所有重复，则用union all代替union

(select course_id
from section
where semester = 'Fall' and year = 2009)
union
(select course_id
from section
where semester = 'Spring' and year = 2010)

交运算intersect

intersect自动去除重复，如果想保留所有重复，则用intersect all代替intersect

(select course_id
from section
where semester = 'Fall' and year = 2009)
intersect
(select course_id
from section
where semester = 'Spring' and year = 2010)

差运算

except在操作前自动去除输入重复，如果想保留所有重复，则用except all代替except

(select course_id
from section
where semester = 'Fall' and year = 2009)
except
(select course_id
from section
where semester = 'Spring' and year = 2010)