我正在努力了解LINQ如何转换为SQL。
我要使用LINQ生成以下查询。
SELECT [OrganizationId]
,[DepartmentId]
,[LocationName]
,[Group1]
,[Group2]
,[Group3]
,[BooklistId]
,[BooklistName]
,COUNT(DISTINCT [OrdererId])
,MAX([ExpectedDate])
FROM [Orders]
WHERE ([DepartmentId] IS NOT NULL AND ([DepartmentId] = '<Guid>')) AND ([Schoolyear] = '2018')
GROUP BY [OrganizationId]
,[DepartmentId]
,[LocationName]
,[Group1]
,[Group2]
,[Group3]
,[BooklistId]
,[BooklistName]
ORDER BY [BooklistName]
使用索引,此查询将在200毫秒内执行。
我的LINQ查询如下:
await _context
.Orders
.Where(i => i.DepartmentId != null && i.DepartmentId.Equals(Parameters.DepartmentId))
.Where(i => i.SchoolYear.Equals(Parameters.SchoolYear))
// Group the data.
.GroupBy(orders => new
{
orders.BooklistId,
orders.BooklistName,
orders.OrganizationId,
orders.DepartmentId,
orders.LocationName,
orders.Groep1,
orders.Groep2,
orders.Groep3
})
.OrderBy(i => i.Key.BooklistName)
.Select(i => new BookListViewModel
{
Count = i.Select(orders => orders.OrdererId).Distinct().Count(s => s != null),
Id = i.Key.OrganizationId,
Name = i.Key.BooklistName,
LocationName = i.Key.LocationName,
Number = i.Key.BooklistId,
Group1 = i.Key.Group1,
Group2 = i.Key.Group2,
Group3 = i.Key.Group3,
DepartmentId = i.Key.DepartmentId,
ExpectedDate = i.Max(orders => orders.ExpectedDate)
})
.ToListAsync();
但是我一直看到:
Microsoft.EntityFrameworkCore.Query:警告:LINQ表达式'GroupBy(new <> f__AnonymousType1`8(BooklistId = [i] .BooklistId,BooklistName = [i] .BooklistName,OrganizationId = [i] .OrganizationId,DepartmentId = [i] .DepartmentId,LocationName = [i] .LocationName,Group1 = [i] .Group1,Group2 = [i] .Group2,Group3 = [i] .Group3),[i])'无法翻译,将在本地进行评估。
Microsoft.EntityFrameworkCore.Query:警告:LINQ表达式'Distinct()'无法翻译,将在本地进行评估。
Microsoft.EntityFrameworkCore.Query:警告:LINQ表达式'where([s]!= null)'无法翻译,将在本地进行评估。
Microsoft.EntityFrameworkCore.Query:警告:LINQ表达式'Count()'无法翻译,将在本地进行评估。
谁能告诉我为什么LINQ查询在内存中执行?我需要在LINQ查询中进行哪些更改才能获得所需的结果?
答案 0 :(得分:1)
查询正在内存中执行,因为您使用语句实例化了BookListViewModel对象的集合
.Select(i => new BookListViewModel
{...})
如果仅删除类BookListViewModel,Linq将在db端执行执行查询(这是个好主意,因为优化器效率更高)...
.Select(i => new
{
Count = i.Select(orders => orders.OrdererId).Distinct().Count(s => s != null),
i.Key.OrganizationId,
i.Key.BooklistName,
i.Key.LocationName,
i.Key.BooklistId,
i.Key.Group1,
i.Key.Group2,
i.Key.Group3,
i.Key.DepartmentId,
ExpectedDate = i.Max(orders => orders.ExpectedDate)
})
然后,您可以在最后实例化您的集合,以便使整个过程看起来像这样……
await _context
.Orders
.Where(i => i.DepartmentId != null && i.DepartmentId.Equals(Parameters.DepartmentId))
.Where(i => i.SchoolYear.Equals(Parameters.SchoolYear))
// Group the data.
.GroupBy(orders => new
{
orders.BooklistId,
orders.BooklistName,
orders.OrganizationId,
orders.DepartmentId,
orders.LocationName,
orders.Group1,
orders.Group2,
orders.Group3
})
.OrderBy(i => i.Key.BooklistName)
.Select(i => new
{
Count = i.Select(orders => orders.OrdererId).Distinct().Count(s => s != null),
i.Key.OrganizationId,
i.Key.BooklistName,
i.Key.LocationName,
i.Key.BooklistId,
i.Key.Group1,
i.Key.Group2,
i.Key.Group3,
i.Key.DepartmentId,
ExpectedDate = i.Max(orders => orders.ExpectedDate)
})
.Select(i => new BookListViewModel
{
Count = i.Count,
Id = i.Id,
Name = i.Name,
LocationName = i.LocationName,
Number = i.Number ,
Group1 = i.Group1 ,
Group2 = i.Group2,
Group3 = i.Group3,
DepartmentId = i.DepartmentId,
ExpectedDate = i.ExpectedDate
})
.ToListAsync();