LINQ to Entity:多个连接条件

Question

关于LINQ和多个连接的帖子很多.但是,我没有找到任何我想要加入的解决方案.

SQL等价物将是这样的:

SELECT * FROM table1 a
LEFT JOIN table2 b ON a.col1 = b.key1 AND
a.col2 = b.key2 AND
b.from_date <= now() AND
b.deleted = 0;

这是我尝试过的众多linq查询之一

var query = (from x in context.table1
             join y in context.table2 on new {x.col1, x.col2} equals {b.key1, b.key2} 
             into result
             from result......

我怎样才能添加日期和删除标志的附加条件？如果我使用.Where条件,则将其视为内连接,而不是左连接.

Answer 1

另一种方式可能是

var query = (from x in context.table1 
             join y in context.table2 on 
             new  {
                  Key1 = x.col1, 
                  Key2 = x.col2,
                  Key3 = true,
                  Key4 = true
                 }
             equals
             new {
                  Key1 = y.key1, 
                  Key2 =  y.key2,
                  Key3 = y.from_date< DateTime.Now,
                  Key4 = !y.deleted
                 }  
             into result
from r in result.DefaultIfEmpty()
select new  {x.Something, r.Something}

Answer 2

LINQ支持连接语法和旧的ANSI-82 WHERE语法.使用后者,您可以在内部联接上执行您要查找的内容

var nowTime = DateTime.Now;
var query = from a in context.table1
            from b in context.table2
            where a.col1 == b.key1
                 && a.col2 == b.key2 
                 && b.from_date < nowTime
                 && b.deleted == false
            select ???;

对于外连接,我更喜欢使用where和select多种语言的混合语法.(意识到LINQ查询中的顺序不需要模仿您在SQL中执行的操作,并且顺序更灵活.)

var nowTime = DateTime.Now;
var query = from b in context.table2
            from a1 in a.Where(a2 => 
                b.key1 = a.col && 
                b.key2 = a.col2 &&
                b.from_date < nowTime &&
                b.deleted == false).DefaultIfEmpty()
            select ???;

Answer 3

我在匿名对象中命名属性时遇到问题：

var subscriptions = context.EmailSubscription.Join(context.EmailQueue,
                    es => new { es.Id, 9 },
                    eq => new { eq.EmailSubscriptionId, eq.EmailTemplateId },
                    (es, eq) => new { es.Id, eq.Id }
                ).ToList();

编译器不高兴，所以上面的答案可以帮助我找出问题所在，这是我的工作解决方案。我花了一些时间才发现愚蠢的错误:)：

var subscriptions = context.EmailSubscription.Join(context.EmailQueue,
                    es => new { EmailSubscriptionId = es.Id, EmailTemplateId  = 9 },
                    eq => new { eq.EmailSubscriptionId, eq.EmailTemplateId },
                    (es, eq) => new { es.Id, eq.Id }
                ).ToList();