https://www.cnblogs.com/id-tangrenhui/p/11647785.html
Go to https://blog.csdn.net/DeMonliuhui/article/details/52988894
Cartesian product: cross join
1-Cartesian product connection: that is, without any conditions, the result set of M*N is reached. 2 --The results of the following two queries are the same. 3 select * from student s cross join class c; 4 select * from student,class;
Descartes result:
Note: If the cross join is added with the where s.classid=c.id condition, it will produce the same result as the self-join:
1 --Add the condition to produce the same result as self-join. 2 select * from student s cross join class c where s.classid=c.id;
Cross join connection result of self-join result set
