select course_id
from section as S
where semester = ’Fall’ and year= 2009
and exists (select *
from section as T
where semester = ’Spring’ and year = 2010 and S.course_id = T.course id);
DBMS如何在内部执行此类查询?我假设内部和外部子查询是独立执行的。我的假设正确吗?如果是,那么子查询在这里如何关联->“ S.course_id = T.course id”?内部关联如何引用外部关联?
这是部分关系的模式:
create table section
(
course_id varchar (8),
sec_id varchar (8),
semester varchar (6),
year numeric (4,0),
building varchar (15),
room number varchar (7),
time slot_id varchar (4),
primary key (course_id, sec_id, semester, year),
foreign key (course_id) references course
);