SELECT title,name FROM Lessons,Users WHERE Lessons.author = Users.email;

和

SELECT title,name FROM Lessons JOIN Users ON Lessons.author = Users.email;

和

SELECT title,name FROM Lessons INNER JOIN Users ON Lessons.author = Users.email;

Lessons有一个名为author的列,索引为Users.email的外键. title是Lessons中的一列,name是Users中的一列