创建两个表：student和teacher
内连接：student和teacher表中相同的记录会显示 select u.id,u.name,u.class_name p.name from student as u inner join teacher as p on u.class_name = p.class_name;
左连接：student表中被查询的字段会被全部显示出来，而teacher表中，只有与表student的classe_name相同的条目的相应字段才会被显示出来 select u.id,u.name,u.class_name p.name from student as u left join teacher as p on u.class_name = p.class_name;
右连接：teacher表中被查询的字段会被全部显示出来，而student表中，只有与表teacher的classe_name相同的条目的相应字段才会被显示出来。 select u.id,u.name,u.class_name p.name from student as u right join teacher as p on u.class_name = p.class_name;
交叉连接：cross join 产生的是笛卡尔集，也就是 M*N 的集合
联合查询：将多表查询结果合并在 一起 select name from student union select name from teacher;
