INNER JOIN（内连接,或等值连接）：获取两个表中字段匹配关系的记录。
LEFT JOIN（左连接）：获取左表所有记录，即使右表没有对应匹配的记录。
RIGHT JOIN（右连接）： 与 LEFT JOIN 相反，用于获取右表所有记录，即使左表没有对应匹配的记录。

创建两个表

SELECT * FROM tb1;

SELECT * FROM tb2;

INNER JOIN(也可以省略 INNER 使用 JOIN，效果一样)

SELECT a.id, a.author, b.count FROM tb1 a INNER JOIN tb2 b ON a.author = b.author;
等价于：
SELECT a.id, a.author, b.count FROM tb1 a, tb2 b WHERE a.author = b.author;

LEFT JOIN

SELECT a.id, a.author, b.count FROM tb1 a LEFT JOIN tb2 b ON a.author = b.author;

RIGHT JOIN

SELECT a.id, a.author, b.count FROM tbl a RIGHT JOIN tb2 b ON a.author = b.author;