Nemo521echo的博客

题目 select title ,count(distinct emp_no) as t from titles group by title having count(distinct emp_no)>=2

#在mysql中添加外键约束时出现ERROR 1215 (HY000): Cannot add foreign key constraint，原因时 在被参照表中没有设置主键(primary key),使得参照表不能添加外键。给teacher 表中tid设置为primary key就可以添加外键约束了 create table course ( cid int(3), cname varchar(...