[英]How to simplify a complex mysql full outer join
我试图在Mysql中创建一个完全外部联接。 我找到了基本问题的几个答案,并且正在使用“联合”使其正常工作。 但是,如果不依靠创建一些临时表,就无法获得正确的语法。 我尝试不使用表来生成查询,但是我始终无法获得结果来包含带有空partner_id的条目。
这是减少的数据集,已经由meeting_id过滤:
+-----+---------+--------+------------+------------+
| pid | first | gender | meeting_id | partner_id |
+-----+---------+--------+------------+------------+
| 2 | Vicki | F | 74 | NULL |
| 54 | Fazal | M | 74 | 4 |
| 4 | Lisa | F | 74 | 54 |
| 10 | Rod | M | 74 | 57 |
| 57 | Kellee | F | 74 | 10 |
| 11 | Jake | M | 74 | 55 |
| 55 | Rosa | F | 74 | 11 |
| 47 | Ralph | M | 74 | 46 |
| 46 | Holly | F | 74 | 47 |
| 40 | Wes | M | 74 | 12 |
| 12 | Lori | F | 74 | 40 |
| 5 | Richard | M | 74 | 6 |
| 6 | Rita | F | 74 | 5 |
| 15 | John | M | 74 | 16 |
| 16 | Corie | F | 74 | 15 |
+-----+---------+--------+------------+------------+
我的原始查询如下所示:
set @mtg=74;
select
a.pid,
concat(a.first, ' ', a.last) as guy,
a.issub as guysub,
b.pid,
concat(b.first, ' ', b.last) as gal,
b.issub as galsub,
b.partner_id
from
scheduled_players a
left outer join
scheduled_players b
on a.partner_id = b.pid
where
a.gender = 'M' and a.meeting_id = @mtg and b.meeting_id = @mtg
union
select
a.pid,
concat(a.first, ' ', a.last) as guy,
a.issub as guysub,
b.pid,
concat(b.first, ' ', b.last) as gal,
b.issub as galsub,
b.partner_id
from
scheduled_players a
left outer join
scheduled_players b
on b.partner_id = a.pid
where
a.gender = 'M' and a.meeting_id = @mtg and b.meeting_id = @mtg
;
该查询未返回带有空partner_id的单个条目。 我在StackOverflow上阅读了许多答案,似乎where子句可能导致外部联接恢复为内部联接。 就我而言,我没有看到这种情况如何发生,但是为了测试这一点,我决定创建临时表以包含“ where”子句元素。 我需要为“ guys”和“ gals”中的每个人创建2个临时表,因为我在查询中有2次表。 结果在这里:
set @mtg=74;
create temporary table if not exists
meeting_guys as select * from scheduled_players
where meeting_id = @mtg and gender='M';
create temporary table if not exists
meeting_gals as select * from scheduled_players
where meeting_id = @mtg and gender='F';
create temporary table if not exists
meeting_guys2 as select * from scheduled_players
where meeting_id = @mtg and gender='M';
create temporary table if not exists
meeting_gals2 as select * from scheduled_players
where meeting_id = @mtg and gender='F';
select
a.pid,
concat(a.first, ' ', a.last) as guy,
a.issub as guysub,
b.pid,
concat(b.first, ' ', b.last) as gal,
b.issub as galsub,
b.partner_id
from
meeting_guys a
left outer join
meeting_gals b
on a.partner_id = b.pid
union
select
a.pid,
concat(a.first, ' ', a.last) as guy,
a.issub as guysub,
b.pid,
concat(b.first, ' ', b.last) as gal,
b.issub as galsub,
b.partner_id
from
meeting_guys2 a
right outer join
meeting_gals2 b
on b.partner_id = a.pid
;
事实证明这是可行的,并且我收到了预期的结果(我删除了姓氏,因为这些是真实的人):
+------+---------+--------+------+--------+--------+------------+
| pid | guy | guysub | pid | gal | galsub | partner_id |
+------+---------+--------+------+--------+--------+------------+
| 54 | Fazal | 0 | 4 | Lisa | 0 | 54 |
| 10 | Rod | 0 | 57 | Kellee | 0 | 10 |
| 11 | Jake | 0 | 55 | Rosa | 0 | 11 |
| 47 | Ralph | 0 | 46 | Holly | 0 | 47 |
| 40 | Wes | 0 | 12 | Lori | 0 | 40 |
| 5 | Richard | 0 | 6 | Rita | 0 | 5 |
| 15 | John | 0 | 16 | Corie | 0 | 15 |
| NULL | NULL | NULL | 2 | Vicki | 0 | NULL |
+------+---------+--------+------+--------+--------+------------+
我可以得到想要的结果,但是我不明白为什么先前的查询不起作用。 幸运的是,我有一个可行的解决方案,但是我真的很想知道是否有更好,更优化的方法。
首先要指出的是,这未经测试,因此您可能只需要对其进行调整,但听起来比修复奇数错误的能力还强。 如果您确实需要我弄清楚为什么我做了某件事,或者您需要我来解决某件事,只需说一句话。
为了解释为什么您的第一次尝试意外消除了空记录,您正确的是您的where子句正在执行此操作。 对于左连接, a.meeting_id = @mtg and b.meeting_id = @mtg
可以使用a.meeting_id = @mtg和(b.meeting_id = @mtg或b.meeting_id为null)代替a.meeting_id = @mtg and b.meeting_id = @mtg
加入后,您将在左侧表格中检查null。
至于替代解决方案,我使用了一个临时表将结果集限制为仅匹配的Meeting_id的早期(以提高性能),以防您的表很大,然后在派生表中筛选M / F。
希望对您有帮助。
set @mtg=74;
create temporary table if not exists
meeting as
select
pid,
concat(first, ' ', last) as full_name,
issub,
partner_id,
meeting_id,
gender
from scheduled_players
where meeting_id = @mtg;
select
M.pid,
M.full_name as guy,
M.issub as guysub,
F.pid,
F.full_name as gal,
F.issub as galsub,
F.partner_id
from
(select * from meeting where gender = 'M') M
left outer join (select * from meeting where gender = 'F') F
on M.partner_id = F.pid
UNION
select
M.pid,
M.full_name as guy,
M.issub as guysub,
F.pid,
F.full_name as gal,
F.issub as galsub,
F.partner_id
from
(select * from meeting where gender = 'M') M
right outer join (select * from meeting where gender = 'F') F
on F.partner_id = M.pid
编辑
如果性能不是问题,那么也许完全忘记temp表,然后直接在派生表中引用该表就更简单了;
select concat(first, ' ', last) as full_name, * from scheduled_players where gender = 'M' and meeting_id = @mtg
select concat(first, ' ', last) as full_name, * from scheduled_players where gender = 'F' and meeting_id = @mtg
您还可以创建一个临时表,然后在单独的查询中插入和更新该临时表。
一天结束时对您有用的一切。
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