有没有办法可以重写这个查询,使其不是相关子查询?
var query = (from o in dbcontext.Orders
let lastStatus = o.OrderStatus.Where(x => x.OrderId == o.Id).OrderByDescending(x => x.CreatedDate).FirstOrDefault()
where lastStatus.OrderId != 1
select new { o.Name, lastStatus.Id }
).ToList();
这导致:
SELECT [o].[Name], (
SELECT TOP(1) [x0].[Id]
FROM [OrderStatus] AS [x0]
WHERE ([x0].[OrderId] = [o].[Id]) AND ([o].[Id] = [x0].[OrderId])
ORDER BY [x0].[CreatedDate] DESC
) AS [Id]
FROM [Orders] AS [o]
WHERE (
SELECT TOP(1) [x].[OrderId]
FROM [OrderStatus] AS [x]
WHERE ([x].[OrderId] = [o].[Id]) AND ([o].[Id] = [x].[OrderId])
ORDER BY [x].[CreatedDate] DESC
) <> 1
我尝试对子查询进行联接,但 EF 2.1 正在做一些奇怪的事情......不是我所期望的;
var query = (from o in dbcontext.Orders
join lastStat in (from os in dbcontext.OrderStatus
orderby os.CreatedDate descending
select new { os }
) on o.Id equals lastStat.os.OrderId
where lastStat.os.StatusId != 1
select new { o.Name, lastStat.os.StatusId }).ToList();
最佳答案
在 EF6 中替换
let x = (...).FirstOrDefault()
与
from x in (...).Take(1).DefaultIfEmpty()
通常会生成更好的 SQL。
所以通常我会建议
var query = (from o in db.Set<Order>()
from lastStatus in o.OrderStatus
.OrderByDescending(s => s.CreatedDate)
.Take(1)
where lastStatus.Id != 1
select new { o.Name, StatusId = lastStatus.Id }
).ToList();
(不需要 DefaultIfEmpty
(左连接),因为 where
条件无论如何都会将其转换为内连接)。
不幸的是,目前(EF Core 2.1.4)存在翻译问题,因此上述内容导致客户评估。
当前的解决方法是将导航属性访问器 o.OrderStatus
替换为相关子查询:
var query = (from o in db.Set<Order>()
from lastStatus in db.Set<OrderStatus>()
.Where(s => o.Id == s.OrderId)
.OrderByDescending(s => s.CreatedDate)
.Take(1)
where lastStatus.Id != 1
select new { o.Name, StatusId = lastStatus.Id }
).ToList();
它为 SqlServer 数据库生成以下 SQL(横向连接):
SELECT [o].[Name], [t].[Id] AS [StatusId]
FROM [Orders] AS [o]
CROSS APPLY (
SELECT TOP(1) [s].*
FROM [OrderStatus] AS [s]
WHERE [s].[OrderId] = [o].[Id]
ORDER BY [s].[CreatedDate] DESC
) AS [t]
WHERE [t].[Id] <> 1
关于c# - 重写 ef core 2.1 中的相关子查询,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/52907947/