我使用的是Asp.NET＆amp;使用SQL Server作为数据库的实体框架，不知何故我得到了这个奇怪的问题
我有这段代码：
var pricingInfo = (from price in invDB.Pricing.AsNoTracking()
join priceD in invDB.PricingDetail.AsNoTracking() on price.PricingId equals priceDtl.PricingId
join tagD in invDB.PricingTagDetail.AsNoTracking() on priceDtl.PricingDetailId equals tagDtl.PricingDetailId
join it in invDB.Item.AsNoTracking() on tagDtl.ItemId equals item.ItemId
join par in invDB.Party.AsNoTracking() on tagDtl.PartyId equals party.PartyId
join b in invDB.Brand.AsNoTracking() on tagDtl.BrandId equals brd.BrandId into t from brand in t.DefaultIfEmpty()
where tagDtl.AvailableQuantity > 0m && price.PricingNo == printNumber
select new
{
TagNo = tagDtl.TagNo,
SellingRate = tagDtl.SellingRate,
Quantity = tagDtl.AvailableQuantity ?? 0m,
ItemCode = item.Name,
UOMId = priceDtl.UOMId,
Brand = brand.BrandCode,
Supplier = party.PartyCode,
Offer = tagDtl.Offer
}).ToList();
使用子查询生成以下sql查询，没有where条件，它从大容量数据中提取完整记录。这会导致大量内存消耗和性能问题。
SELECT
[Filter1].[PricingId1] AS [PricingId],
[Filter1].[TagNo] AS [TagNo],
[Filter1].[SellingRate1] AS [SellingRate],
CASE WHEN ([Filter1].[AvailableQuantity] IS NULL) THEN cast(0 as decimal(18)) ELSE [Filter1].[AvailableQuantity] END AS [C1],
[Filter1].[Name] AS [Name],
[Filter1].[UOMId 1] AS [UOMId ],
[Extent6].[BrandCode] AS [BrandCode],
[Filter1].[PartyCode] AS [PartyCode],
[Filter1].[Offer] AS [Offer]
FROM
(
SELECT [Extent1].[PricingId] AS [PricingId1], [Extent1].[PricingNo] AS [PricingNo], [Extent2].[UnitOfMeasurementId] AS [UnitOfMeasurementId1], [Extent3].[TagNo] AS [TagNo], [Extent3].[BrandId] AS [BrandId1], [Extent3].[SellingRate] AS [SellingRate1], [Extent3].[AvailableQuantity] AS [AvailableQuantity], [Extent3].[Offer] AS [Offer], [Extent4].[Name] AS [Name], [Extent5].[PartyCode] AS [PartyCode]
FROM [PanERP].[Pricing] AS [Extent1]
INNER JOIN [PanERP].[PricingDetail] AS [Extent2] ON [Extent1].[PricingId] = [Extent2].[PricingId]
INNER JOIN [PanERP].[PricingTagDetail] AS [Extent3] ON [Extent2].[PricingDetailId] = [Extent3].[PricingDetailId]
INNER JOIN [PanERP].[Item] AS [Extent4] ON [Extent3].[ItemId] = [Extent4].[ItemId]
INNER JOIN [PanERP].[Party] AS [Extent5] ON [Extent3].[PartyId] = [Extent5].[PartyId]
WHERE [Extent3].[AvailableQuantity] > cast(0 as decimal(18))
) AS [Filter1]
LEFT OUTER JOIN [PanERP].[Brand] AS [Extent6] ON [Filter1].[BrandId1] = [Extent6].[BrandId]
WHERE ([Filter1].[PricingNo] = @p__linq__0) OR (([Filter1].[PricingNo] IS NULL) AND (@p__linq__0 IS NULL))
但是当我改变条件时
其中tagDtl.AvailableQuantity＆gt; 0米
作为变量，它创建了另一个没有嵌套select语句的SQL查询。
这是修改后的代码
decimal availableQuantity = 0m;
var pricingInfo = (from price in invDB.Pricing.AsNoTracking()
join priceD in invDB.PricingDetail.AsNoTracking() on price.PricingId equals priceDtl.PricingId
join tagD in invDB.PricingTagDetail.AsNoTracking() on priceDtl.PricingDetailId equals tagDtl.PricingDetailId
join it in invDB.Item.AsNoTracking() on tagDtl.ItemId equals item.ItemId
join par in invDB.Party.AsNoTracking() on tagDtl.PartyId equals party.PartyId
join b in invDB.Brand.AsNoTracking() on tagDtl.BrandId equals brd.BrandId into t from brand in t.DefaultIfEmpty()
where tagDtl.AvailableQuantity > availableQuantity && price.PricingNo == printNumber
select new
{
TagNo = tagDtl.TagNo,
SellingRate = tagDtl.SellingRate,
Quantity = tagDtl.AvailableQuantity ?? availableQuantity,
ItemCode = item.Name,
UOMId = priceDtl.UOMId,
Brand = brand.BrandCode,
Supplier = party.PartyCode,
Offer = tagDtl.Offer
}).ToList();
这是没有嵌套SQL语句的SQL查询。
SELECT
[Extent1].[PricingId] AS [PricingId],
[Extent3].[TagNo] AS [TagNo],
[Extent3].[SellingRate] AS [SellingRate],
CASE WHEN ([Extent3].[AvailableQuantity] IS NULL) THEN cast(0 as decimal(18)) ELSE [Extent3].[AvailableQuantity] END AS [C1],
[Extent4].[Name] AS [Name],
[Extent2].[UOMId ] AS [UOMId ],
[Extent6].[BrandCode] AS [BrandCode],
[Extent5].[PartyCode] AS [PartyCode],
[Extent3].[Offer] AS [Offer]
FROM [PanERP].[Pricing] AS [Extent1]
INNER JOIN [PanERP].[PricingDetail] AS [Extent2] ON [Extent1].[PricingId] = [Extent2].[PricingId]
INNER JOIN [PanERP].[PricingTagDetail] AS [Extent3] ON [Extent2].[PricingDetailId] = [Extent3].[PricingDetailId]
INNER JOIN [PanERP].[Item] AS [Extent4] ON [Extent3].[ItemId] = [Extent4].[ItemId]
INNER JOIN [PanERP].[Party] AS [Extent5] ON [Extent3].[PartyId] = [Extent5].[PartyId]
LEFT OUTER JOIN [PanERP].[Brand] AS [Extent6] ON [Extent3].[BrandId] = [Extent6].[BrandId]
WHERE ([Extent3].[AvailableQuantity] > @p__linq__0) AND (([Extent1].[PricingNo] = @p__linq__1) OR (([Extent1].[PricingNo] IS NULL) AND (@p__linq__1 IS NULL)))
如果我将where条件移动到模型定义作为lambda表达式，就像这样
来自inventoryDb.Pricing.AsNoTracking（）中的价格。其中（c =＆gt; c.PricingNo == printNumber））
然后它也可以正常工作。
为什么LINQ会生成嵌套的Select？我们怎样才能避免这种情况？
提前感谢您的回答。
答案 0 :(得分：0)
好吧，我想您已经对您的评论回答了自己的问题。我将尝试澄清发生了什么。
当您使用诸如0m之类的硬编码常量时，框架会将其转换为SQL，将值保持为常量：
WHERE [Extent3].[AvailableQuantity] > cast(0 as decimal(18))
当您使用局部变量（如“ availableQuantity”）时，框架会创建一个参数：
([Extent3].[AvailableQuantity] > @p__linq__0)
我可能是错的，但是，正如我所看到的，这样做是为了在编写代码（常数=常数，变量=参数）时保持程序员的目标。
那子查询呢？
这是一种查询优化逻辑（至少在这种情况下可能是一个糟糕的逻辑）。使用参数进行查询时，可能会运行几次，但是SQL Server将始终使用相同的执行计划，从而使查询速度更快。使用常量时，每个查询都需要重新评估（如果您选中SQL Server活动监视器，则无论参数值如何，带有参数的查询都会被视为同一查询）。
我认为这样（很抱歉，我找不到关于它的任何文档），实体框架正在尝试隔离查询。外部/通用参数（使用参数）和内部/特定参数（使用常量）。
如果有人可以补充一些与此主题相关的Microsoft文档，我将很高兴...