oracle左外连接没有显示正确的空值

时间:2008-12-12 11:58:58

标签: oracle join

我遇到了在oracle中创建查询的问题,该问题似乎并不想加入缺失值

我拥有的表是:

table myTable(refnum, contid, type)

values are:
1, 10, 90000
2, 20, 90000
3, 30, 90000
4, 20, 10000
5, 30, 10000
6, 10, 20000
7, 20, 20000
8, 30, 20000

我所追求的领域的分解是这样的:

select a.refnum from myTable a where type = 90000
select b.refnum from myTable b where type = 10000 and contid in (select contid from myTable where type = 90000)
select c.refnum from myTable c where type = 20000 and contid in (select contid from myTable where type = 90000)

我之后的查询结果是:

a.refnum, b.refnum, c.refnum

我认为这样可行:

select a.refnum, b.refnum, c.refnum
from myTable a 
left outer join myTable b on (a.contid = b.contid) 
left outer join myTable c on (a.contid = c.contid) 
where a.id_tp_cd = 90000
and b.id_tp_cd = 10000
and c.id_tp_cd = 20000

所以值应该是:

1, null, 6
2, 4, 7
3, 5, 8

但它唯一的回归:

2, 4, 7
3, 5, 8

我认为左连接会显示左边的所有值,并为右边创建一个空值。

帮助:(

2 个答案:

答案 0 :(得分:27)

你说左边连接将返回没有匹配的右边的空值是正确的,但是当你将这个限制添加到where子句时,你不允许返回这些空值:

and b.id_tp_cd = 10000
and c.id_tp_cd = 20000

您应该能够将这些放在连接的“on”子句中,因此只返回右侧的相关行。

select a.refnum, b.refnum, c.refnum
from myTable a 
left outer join myTable b on (a.contid = b.contid and b.id_tp_cd = 10000) 
left outer join myTable c on (a.contid = c.contid and c.id_tp_cd = 20000) 
where a.id_tp_cd = 90000

答案 1 :(得分:2)

或使用Oracle语法而不是ansi

select a.refnum, b.refnum, c.refnum
from myTable a, mytable b, mytable c
where a.contid=b.contid(+)
and a.contid=c.contid(+)
and a.type = 90000
and b.type(+) = 10000
and c.type(+) = 20000;


REFNUM     REFNUM     REFNUM
---------- ---------- ----------
     1                     6
     2          4          7
     3          5          8
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