如何实现左连接在JOIN扩展方法 [英] How to implement left join in JOIN Extension method
问题描述
我想实现一个外连接上这种类型的查询对 p.Person
表。
我将如何做到这一点。
这例子是从 HTTP采取? //ashishware.com/DSLinqExample.shtml
VAR onlyinfo = p.Person
。凡( N => n.FirstName.Contains('A'))
。加入(p.PersonInfo,$ b $亿=> n.PersonId,
M = GT; m.PersonId,
(N,M)=>米)
.ToArray&所述; Persons.PersonInfoRow>();
通常左联接在LINQ与组联接蓝本,有时与 DefaultIfEmpty
和的SelectMany
结合:
VAR leftJoin = p.Person.Where(N => n.FirstName.Contains(A))
.GroupJoin(p.PersonInfo,$ b $亿=> n.PersonId,
M = GT; m.PersonId,
(N,MS)=>新建{N,MS = ms.DefaultIfEmpty()})
.SelectMany(Z => ; z.ms.Select(M = gt;新建{N =锌,M));
这将使对(N,M),其中 n的序列
是 p.Person
和 M
是 p.PersonInfo
,但 M
如果没有匹配的将是空的。
(这是完全未经测试,顺便说一句 - 但无论如何应该给你的想法:)
I am trying to implement an outer join on this kind of query for the p.Person
table.
How would I do this?
This example is taken from http://ashishware.com/DSLinqExample.shtml
var onlyinfo = p.Person
.Where(n => n.FirstName.Contains('a'))
.Join(p.PersonInfo,
n => n.PersonId,
m => m.PersonId,
(n, m) => m)
.ToArray<Persons.PersonInfoRow>();
Normally left joins in LINQ are modelled with group joins, sometimes in conjunction with DefaultIfEmpty
and SelectMany
:
var leftJoin = p.Person.Where(n => n.FirstName.Contains("a"))
.GroupJoin(p.PersonInfo,
n => n.PersonId,
m => m.PersonId,
(n, ms) => new { n, ms = ms.DefaultIfEmpty() })
.SelectMany(z => z.ms.Select(m => new { n = z.n, m ));
That will give a sequence of pairs (n, m) where n
is the entry from p.Person
and m
is the entry from p.PersonInfo
, but m
will be null if there are no matches.
(It's completely untested, btw - but should give you the idea anyway :)
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