1.内连接(等值连接 )

等值连接

select * from t_class c,t_teacher t where c.class_teacher = t.tea_id

内连接

select * from t_class c inner join t_teacher t on c.class_teacher = t.tea_id

2.外连接

左外

select * from t_class c left join t_teacher t on c.class_teacher = t.tea_id

右外

select * from t_teacher t right join t_class c on c.class_teacher = t.tea_id

全外（左 || 右） 将永远的返回主(左)表以及从(右)表当中的所有记录

select * from t_class c full outer join t_teacher t on c.class_teacher = t.tea_id

select * from t_class c left join t_teacher t on c.class_teacher = t.tea_id union select * from t_class c right join t_teacher t on c.class_teacher = t.tea_id

3.使用子查询

select c.class_id,c.class_code,c.class_name,c.class_open_date, (select t.tea_name from t_teacher t where t.tea_id = c.class_teacher) as teacher_name, (select t.tea_sex from t_teacher t where t.tea_id = c.class_teacher) as teacher_sex, (select t.tea_phone from t_teacher t where t.tea_id = c.class_teacher) as teacher_phone from t_class c