左连接：以左表为主
select a.*,b.* from a left join b on a.b_id = b.id;
右连接：以右表为主
select a.*,b.* from a right join b on a.b_id = b.id;
内连接：只列出两张表关联查询符合条件的记录
select a.*,b.* from a inner join b on a.b_id = b.id;
案例：
select c.id c_id,c.`name` c_name
from t_teacher t LEFT JOIN t_class c on t.id=c.t_id; #4条，以老师表为主
select c.id c_id,c.`name` c_name
from t_teacher t RIGHT JOIN t_class c on t.id=c.t_id; #4条，以班级表为主
select c.id c_id,c.`name` c_name
from t_teacher t INNER JOIN t_class c on t.id=c.t_id; #3条，只展示匹配条件的记录