blocks|key|174324|text|假设在您的类中有一个关于City或CityID的[Required]约束。假设存在没有(有效)+City的Person记录。满足[Required]的唯一方法是执行内部联接。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|174325|但只要你的数据库和模型中的约束匹配(即CityID+INT+NOT+NULL)，使用哪种类型的连接就无关紧要了。这应该是正常的情况。|174326|如果没有约束，你当然会期望一个左连接。|174327|entityMap^0|O|A|1S|A|0|J|J|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@$9|M|A|N|B|C]|$9|O|A|P|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|Q|8|@$9|R|A|S|B|C]]|D|@]|E|$]]|$1|H|3|I|5|6|7|T|8|@]|D|@]|E|$]]|$1|J|3|-4|5|6|7|U|8|@]|D|@]|E|$]]]|K|$]]

Suppose that in your class there is a <code>[Required]</code> constraint on City or CityID. And suppose there are Person records without a (valid) City. The only way to satisfy the <code>[Required]</code> is to perform an inner join. 

But as long as the constraints in your Db and model match (ie <code>CityID INT NOT NULL</code>) it wouldn't really matter what kind of Join is used. That should be the normal case. 

And without the constraint you would of course expect a Left Join.

blocks|key|174329|text|我知道这是一个老问题，但是如果其他人像我一样使用EF+Code+First，我的问题是流畅的映射：|type|unstyled|depth|inlineStyleRanges|offset|length|style|BOLD|entityRanges|data|174330|++++protected+override+void+OnModelCreating(DbModelBuilder+modelBuilder)
++++{
++++++++base.OnModelCreating(modelBuilder);

++++++++modelBuilder.Entity<Parent>()
++++++++++++.HasOptional(a+=>+a.Child)+/*+LEFT+OUTER+JOIN+*/
++++++++++++.WithMany()
++++++++++++.HasForeignKey(a+=>+a.ChildId);
++++}|code-block|syntax|javascript|174331|被转换为左外部联接，而|174332|++++modelBuilder.Entity<Parent>()
++++++++.HasRequired(a+=>+a.Child)+/*+INNER+JOIN+*/
++++++++.WithMany()
++++++++.HasForeignKey(a+=>+a.ChildId);|174333|被转换为内部联接。|174334|entityMap^0|O|D|0|0|4|5|0|0|4|4|0^^$0|@$1|2|3|4|5|6|7|S|8|@$9|T|A|U|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|V|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|W|8|@$9|X|A|Y|B|C]]|D|@]|E|$]]|$1|M|3|N|5|H|7|Z|8|@]|D|@]|E|$I|J]]|$1|O|3|P|5|6|7|10|8|@$9|11|A|12|B|C]]|D|@]|E|$]]|$1|Q|3|-4|5|6|7|13|8|@]|D|@]|E|$]]]|R|$]]

I know this is an old question, but if anyone else lands here using EF Code First as I am, my issue was in the fluent mappings:

<pre><code> protected override void OnModelCreating(DbModelBuilder modelBuilder)
 {
 base.OnModelCreating(modelBuilder);

 modelBuilder.Entity&lt;Parent&gt;()
 .HasOptional(a =&gt; a.Child) /* LEFT OUTER JOIN */
 .WithMany()
 .HasForeignKey(a =&gt; a.ChildId);
 }
</code></pre>

is translated as a LEFT OUTER JOIN whereas

<pre><code> modelBuilder.Entity&lt;Parent&gt;()
 .HasRequired(a =&gt; a.Child) /* INNER JOIN */
 .WithMany()
 .HasForeignKey(a =&gt; a.ChildId);
</code></pre>

is translated as an INNER JOIN.

As I was investigating the difference between <code>Include</code> and <code>Join</code> I found that : 

If the DB does not include a Foreign Keys -it has no navigation props so it's better to use <code>Join</code>

If It does have a navigation props - then use <code>Include</code>. ( it also save a db hit.)

But one <a href="https://stackoverflow.com/a/12284852/859154">answer here</a> caught my attention: 

<blockquote>
 Include is implemented as a join. Depending on the nullability of the
 included link it is an inner or left join.
</blockquote>

Question : 

How does the nullity affects the left / inner join ? 

In Sql server I can have a <code>Cities</code> table and <code>Persons</code> table and a person can have a <code>NULL</code> <code>CityID</code>.

Why does entity Framework decides for me what kind of join it is ?

<h1>edit : visualization :</h1>

<img src="https://i.stack.imgur.com/wMFGL.png" alt="enter image description here">

<img src="https://i.stack.imgur.com/I6dYU.png" alt="enter image description here">

Now lets change CityId to not null : 

<img src="https://i.stack.imgur.com/Y5KaM.png" alt="enter image description here">

And here is the change : 

<img src="https://i.stack.imgur.com/ZrOr1.png" alt="enter image description here">

Entity framework Include command - Left or inner join?

数据库

在研究Include和Join之间的差异时，我发现：如果数据库不包含外键，那么最好使用Join，因为-it没有导航道具如果它确实有导航道具，那就使用Include。(它还可以保存数据库命中。)但是有一个引起了我的注意： Include是作为联接实现的。根据所包含链接的可空性，它是内部或左连接。问题:空性如何影响左/内连...

问实体框架包含命令-左连接还是内连接？
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