我有一个SQL查询与这样的Lambda表达式,通常有比这个例子更多的连接.
select Table2.a,Table2.b,Table2.c,Table2.d
from Table1
LEFT OUTER JOIN Table2
ON Table2.a = Table1.a and
Table2.b = Table1.b and
Table2.c = Table1.c
LEFT OUTER JOIN Table3
ON Table3.b = Table1.b AND
Table3.c = Table1.c AND
Table3.d = Table1.d
where ( Table1.a = ValueA )
order by Table3.f
我正在使用Join()Lambda Expression执行此操作,但我在sql Server分析器中看到,这会生成一个INNER JOIN,我需要一个LEFT OUTER JOIN.
这是我用Join()做的
var RS = DBContext.Table1.Join(DBContext.Table2,Table1 => new {Table1.a,Table1.b,Table1.c},Table2 => new {Table1.a,(Table1,Table2) => new {Table1})
.Join(DBContext.Table3,LastJoin => new {LastJoin.Table1.b,LastJoin.Table1.c,LastJoin.Table1.d},Table3 => new {Table3.b,Table3.c,Table3.d},(LastJoin,Table3) => new {LastJoin.Table1,Table3})
.Where (LastTable => LastTable.Table1.a == ValueA)
.OrderBy(LastTable => LastTable.Table3.f)
.Select (LastTable => new {LastTable.Table1,LastTable.Table3});
我一直在读它可以使用DefaultIfEmpty()或GroupJoin()完成,但我没有找到任何具有多个LEFT OUTER JOIN的复杂示例.
解决方法
为什么不尝试使用linq查询,与lambda表达式相比,编写和理解它们也容易得多.我有这样的实现,如：
var products =
from p in this.Products
from cat in this.ProductCategoryProducts
.Where(c => c.ProductID == p.ProductID).DefaultIfEmpty()
from pc in this.ProductCategories
.Where(pc => ac.ProductCategoryID == cat.ProductCategoryID).DefaultIfEmpty()
where p.ProductID == productID
select new
{
ProductID = p.ProductID,Heading = p.Heading,Category = pc.ProductCategory
};
return products ;
