原文：http://www.codeproject.com/Articles/33052/Visual-Representation-of-SQL-Joins
从视图上介绍了7种不同类型的JOIN操作：
- INNER JOIN
- LEFT JOIN
- RIGHT JOIN
- OUTER JOIN
- LEFT JOIN EXCLUDING INNER JOIN
- RIGHT JOIN EXCLUDING INNER JOIN
- OUTER JOIN EXCLUDING INNER JOIN
Visual Representation of SQL Joins
原文：https://www.cnblogs.com/huangzejun/p/8142988.html