Linq - 进入leftjoin

Question

from foo in context.Foo
join current in context.v_CurrentFooIds on foo.Id equals
   current.CurrentId
join approved in context.v_ApprovedFooIds on foo.VersionIndependentId
   equals approved.VersionIndependentId into leftjoin
from appr in leftjoin.DefaultIfEmpty()
where
   matches.Contains(foo.UniqueFormulaNumber) && !foo.IsTemplate
   && appr == null

我不明白leftjoin在这种情况下做了什么。

我迷失在equals approved.VersionIndependent into leftJoin from appr in leftjoin.DefaultIfEmpty()。

这是什么意思？