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[数据库]mysql连接查询经典小例题


mysql连接查询:

Mysql连接查询支持多表连接

对同一张表可以重复连接多次(别名在多次连接同一张表时很重要)

例题1:

下面有2张表

teams表

 

 

比赛结果表:result

 

 

问题:

得出一张表:

主队,客队,比赛成绩,比赛时间

方法一:(子查询和连接查询混合)

  step1:

select result.id, t_name as h_name,match_time,result from teams join result on teams.t_id=result.h_id

step2:

select result.id ,t_name as g_name from teams join result on teams.t_id=result.g_id

得到

step3:根据比赛的id 相等连接以上两表即可

复制代码
select t1.id,h_name,g_name,result,match_time from(select result.id, t_name as h_name,match_time,result from teams join result on teams.t_id=result.h_id) as t1 join (select result.id ,t_name as g_name from teams join result on teams.t_id=result.g_id) as t2 on t1.id=t2.id;
复制代码

即可得到

结果是出来了,有点繁琐

方法二:多次连接查询

select result.id,t1.t_name as h_name ,t2.t_name as g_name ,result,match_time from result join teams as t1 on result.h_id=t1.t_id join teams as t2 on t2.t_id=result.g_id;

即可得到:

Teams表要连接2次所以要有别名

 

例题2:

现有下表 subject

 

求这样一个表

父栏目名 ,子栏目名称

连接查询

自己连接自己更需要别名了

 

select t1.name as p_name,t2.name as son_name from subject as t1 join subject as t2 on t1.id=t2.pid;

 

即可得到

 

 

 

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