Mysql 将两列合并为一列,同时保留其他列。然后与另一个表合并
我的桌子:Mysql 将两列合并为一列,同时保留其他列。然后与另一个表合并,mysql,sql,Mysql,Sql,我的桌子: // friendships +-----------------+-----------------+-----------------+ | fid | person1 | person2 | |-----------------+-----------------|-----------------+ +-----------------+-----------------+-----------
// friendships
+-----------------+-----------------+-----------------+
| fid | person1 | person2 |
|-----------------+-----------------|-----------------+
+-----------------+-----------------+-----------------+
| 1 | personid | personid |
|-----------------+-----------------|-----------------+
| 2 | personid | personid |
|-----------------+-----------------|-----------------+
// persons
+-----------------+-----------------+-----------------+
| pid | firstname | lastname |
|-----------------+-----------------|-----------------+
+-----------------+-----------------+-----------------+
| 1 | name | name |
|-----------------+-----------------|-----------------+
| 2 | name | name |
|-----------------+-----------------|-----------------+
1)我想获取Friendlations表中包含某个personid的所有行。此id可以在person1或person2列中。应保留fid
列,但person
列应仅为一列,例如:
Select fid, person1 as person, person2 as person FROM friendships
WHERE person1 = some_personid
OR person2 = some_personid;
(此查询中的2个人列应仅为一个)。我该怎么做
2) 我想加入fid.person=persons.pid上步骤1的结果的persons表 如果我理解正确,您需要所需人员的每个朋友的名字和姓氏,无论此人是
person1
还是person2
在这种情况下,如果友谊关系不是对称的,可以通过子查询来实现
select *
from persons p
join (
select fid, person1 as person, person2 as otherPerson
from friendship
where person1 = 'yourPerson'
union all
select fid, person2 as person, person1 as otherPerson
from friendship
where person2 = 'yourPerson'
) f
on p.pid = f.otherPerson
如果是对称的,查询就容易多了,因为person2
下的每一行都有一个与person1
下的所需人员对应的行
select *
from friendship f
join person p
on f.person2 = p.pid
where person1 = 'yourPerson'
如果我理解正确,您需要所需人员的每个朋友的名字和姓氏,无论此人是
person1
还是person2
在这种情况下,如果友谊关系不是对称的,可以通过子查询来实现
select *
from persons p
join (
select fid, person1 as person, person2 as otherPerson
from friendship
where person1 = 'yourPerson'
union all
select fid, person2 as person, person1 as otherPerson
from friendship
where person2 = 'yourPerson'
) f
on p.pid = f.otherPerson
如果是对称的,查询就容易多了,因为person2
下的每一行都有一个与person1
下的所需人员对应的行
select *
from friendship f
join person p
on f.person2 = p.pid
where person1 = 'yourPerson'
对于第一步,在where筛选器中使用OR子句来指示应该保存哪些记录。然后在select中使用case语句来选择哪个人
select fid
, case person1 when some_personid then person1 else person2 end as person
from friendships
where (person1 = some_personid
or person2 = some_personid)
对于步骤二,您可以从persons中选择,并使用子查询从步骤1连接到表
select *
from persons p
inner join (
select fid
, case person1 when some_personid then person1 else person2 end as person
from friendships
where (person1 = some_personid
or person2 = some_personid)
) f on f.person = p.pid
希望这有帮助 对于第一步,在where筛选器中使用OR子句来指示应保留哪些记录。然后在select中使用case语句来选择哪个人
select fid
, case person1 when some_personid then person1 else person2 end as person
from friendships
where (person1 = some_personid
or person2 = some_personid)
对于步骤二,您可以从persons中选择,并使用子查询从步骤1连接到表
select *
from persons p
inner join (
select fid
, case person1 when some_personid then person1 else person2 end as person
from friendships
where (person1 = some_personid
or person2 = some_personid)
) f on f.person = p.pid
希望这有帮助 友谊关系是对称的吗?我的意思是如果A是B的朋友,B也是A的朋友吗?友谊关系是对称的吗?我的意思是如果A是B的朋友,B也是A的朋友吗?