怎么用sql语句直接检查两个表的对应关系
班级表
ID(主键)，CLASSID(班号), GRADE(年级)
教师-班级对应表
ID(主键),CLASSID(对应班级表的ID),TEACHERID(对应教师表的ID)
怎么直接用sql语句就能判断 某一年级的所有班都至少有一名授课教师？
[解决办法]
select a.classid,count(teacherid)
from class a,teacher b
where a.classid=b.classid(+)
group by a.classid having count(teacherid)>0
select distinct grade "某一个班没有授课老师的年级" from (
select a.grade grade, a.classid ci, count(teacherid) cn
from class a,teacher b
where a.classid=b.classid(+)
group by a.grade, a.classid ) where cn = 0
