实体框架包含命令 - 左连接还是内连接？

Question

As I was investigating the difference between Include and Join I found that :

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

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

But one answer here caught my attention:

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

Question :

How does the nullity affects the left / inner join ?

In Sql server I can have a Cities table and Persons table and a person can have a NULL CityID .

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

edit : visualization :

Now lets change CityId to not null :

And here is the change :

Answer 1

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

But as long as the constraints in your Db and model match (ie CityID INT NOT NULL ) 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.

Answer 2

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:

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);

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

is translated as a LEFT OUTER JOIN whereas

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

is translated as an INNER JOIN .