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

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;

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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