1.将stu表和sc表做连接：

select * from stu left join sc on stu.s_id=sc.s_id;

2.将stu表和sc表和co表做连接：

select * from stu left join sc on stu.s_id=sc.s_id left join co on sc.c_id=co.c_id;

3.将四表做连接：

select * from stu left join sc on stu.s_id=sc.s_id left join co on sc.c_id=co.c_id

left join te on co.t_id=te.t_id;

4.将四表做连接，删除不必要字段：

select stu.*,co.c_id,c_name,score,te.* from stu left join sc on stu.s_id=sc.s_id

left join co on sc.c_id=co.c_id

left join te on co.t_id=te.t_id;

5.将stu表和stu表做纵向链接：

select * from stu union select * from stu;

6.将stu表和stu表做纵向链接（不自动去重）：

select * from stu union all select * from stu;

7.将stu表的s_id、s_name与co表的c_id、c_name做纵向链接：

select s_id,s_name from stu union select c_id,c_name from co;