我有这个架构：
Lists ( ListId, Name, DateCreated, ... )
ListItems( ListId, Text, Foo, Baz, Qux, ... )
我有一个IQueryable<List>代表另一个Linq查询，它返回一些List个实体。
我希望JOIN使用ListItems的一些汇总数据，但这证明很难，因为Linq生成效率低下的SQL - 但我也希望使查询可以组合。
这与我希望Linq生成的SQL类似：
SELECT
*
FROM
(
-- This part represents the IQueryable:
SELECT
ListId,
Name,
...
FROM
Lists
ORDER BY
Lists.DateCreated
OFFSET
0 ROWS FETCH NEXT 25 ROWS ONLY -- Linq's .Skip(0).Take(25)
) AS ListsResult
LEFT JOIN
(
-- This is the aggregate data query I want Linq to generate:
SELECT
ListId,
COUNT(1) AS [Count],
COUNT( CASE WHEN Foo = 'bar' THEN 1 ELSE NULL END ) AS CountFooBar,
COUNT( CASE WHEN Baz > 5 THEN 1 ELSE NULL END ) AS CountBaz5
FROM
ListItems
WHERE
Qux IS NOT NULL
GROUP BY
ListId
) AS ItemsStats ON ListResults.ListId = ItemsStats.ListId
这是我的Linq - 我更喜欢扩展方法语法：
IQueryable lists = GetLists( 0, 25 );
var stats = this.dbContext.ListItems
.Where( (ListItem li) => li.Qux != null )
.GroupBy( (ListItem li) => li.ListId )
.Select( grp => new
{
grp.Key,
Count = grp.Count(),
CountFooBar = grp.Count( (ListItem li) => li.Foo == "bar" )
CountBaz5 = grp.Count( (ListItem li) => li.Baz > 5 )
} )
return lists
.Join(
inner: stats,
outerKeySelector: (List l) => l.ListId,
innerKeySelector: grp => grp.Key,
resultSelector: (list, itemStats) => new { list, itemsStats }
)
然而，这会生成如下所示的SQL（此查询显示我的真实表和列名称，这比我之前发布的模式稍微复杂一点：）
SELECT
[Project13].[C2] AS [C1],
[Project13].[ListId] AS [ListId],
[Project13].[C1] AS [C2],
[Project13].[C3] AS [C3],
[Project13].[C4] AS [C4],
[Project13].[C5] AS [C5],
[Project13].[C6] AS [C6],
[Project13].[C7] AS [C7]
FROM ( SELECT
[Project11].[C1] AS [C1],
[Project11].[ListId] AS [ListId],
[Project11].[C2] AS [C2],
[Project11].[C3] AS [C3],
[Project11].[C4] AS [C4],
[Project11].[C5] AS [C5],
[Project11].[C6] AS [C6],
(SELECT
COUNT(1) AS [A1]
FROM (SELECT [Project12].[ListId] AS [ListId]
FROM ( SELECT
[Extent11].[ListId] AS [ListId],
[Extent11].[Created] AS [Created]
FROM [dbo].[Lists] AS [Extent11]
WHERE ([Extent11].[TenantId] = 8) AND ([Extent11].[BlarghId] = 8)
) AS [Project12]
ORDER BY [Project12].[Created] DESC
OFFSET 0 ROWS FETCH NEXT 25 ROWS ONLY ) AS [Limit6]
INNER JOIN [dbo].[ListItems] AS [Extent12] ON ([Limit6].[TenantId] = [Extent12].[TenantId]) AND ([Limit6].[BlarghId] = [Extent12].[BlarghId]) AND ([Limit6].[ListId] = [Extent12].[ListId])
WHERE (([Extent12].[Baz] > 0) OR ((LEN([Extent12].[Notes])) > 0) OR ((LEN([Extent12].[Value])) > 0)) AND ([Project11].[TenantId] = [Extent12].[TenantId]) AND ([Project11].[BlarghId] = [Extent12].[BlarghId]) AND ([Project11].[ListId] = [Extent12].[ListId]) AND ([Extent12].[RecommendationRevision] IS NOT NULL)) AS [C7]
FROM ( SELECT
[Project9].[C1] AS [C1],
[Project9].[TenantId] AS [TenantId],
[Project9].[BlarghId] AS [BlarghId],
[Project9].[ListId] AS [ListId],
[Project9].[C2] AS [C2],
[Project9].[C3] AS [C3],
[Project9].[C4] AS [C4],
[Project9].[C5] AS [C5],
(SELECT
COUNT(1) AS [A1]
FROM (SELECT [Project10].[TenantId] AS [TenantId], [Project10].[BlarghId] AS [BlarghId], [Project10].[ListId] AS [ListId]
FROM ( SELECT
[Extent9].[TenantId] AS [TenantId],
[Extent9].[BlarghId] AS [BlarghId],
[Extent9].[ListId] AS [ListId],
[Extent9].[Created] AS [Created]
FROM [dbo].[Lists] AS [Extent9]
WHERE ([Extent9].[TenantId] = 8) AND ([Extent9].[BlarghId] = 8)
) AS [Project10]
ORDER BY [Project10].[Created] DESC
OFFSET 0 ROWS FETCH NEXT 25 ROWS ONLY ) AS [Limit5]
INNER JOIN [dbo].[ListItems] AS [Extent10] ON ([Limit5].[TenantId] = [Extent10].[TenantId]) AND ([Limit5].[BlarghId] = [Extent10].[BlarghId]) AND ([Limit5].[ListId] = [Extent10].[ListId])
WHERE (([Extent10].[Baz] > 0) OR ((LEN([Extent10].[Notes])) > 0) OR ((LEN([Extent10].[Value])) > 0)) AND ([Project9].[TenantId] = [Extent10].[TenantId]) AND ([Project9].[BlarghId] = [Extent10].[BlarghId]) AND ([Project9].[ListId] = [Extent10].[ListId]) AND (3 = [Extent10].[Baz])) AS [C6]
FROM ( SELECT
[Project7].[C1] AS [C1],
[Project7].[TenantId] AS [TenantId],
[Project7].[BlarghId] AS [BlarghId],
[Project7].[ListId] AS [ListId],
[Project7].[C2] AS [C2],
[Project7].[C3] AS [C3],
[Project7].[C4] AS [C4],
(SELECT
COUNT(1) AS [A1]
FROM (SELECT [Project8].[TenantId] AS [TenantId], [Project8].[BlarghId] AS [BlarghId], [Project8].[ListId] AS [ListId]
FROM ( SELECT
[Extent7].[TenantId] AS [TenantId],
[Extent7].[BlarghId] AS [BlarghId],
[Extent7].[ListId] AS [ListId],
[Extent7].[Created] AS [Created]
FROM [dbo].[Lists] AS [Extent7]
WHERE ([Extent7].[TenantId] = 8) AND ([Extent7].[BlarghId] = 8)
) AS [Project8]
ORDER BY [Project8].[Created] DESC
OFFSET 0 ROWS FETCH NEXT 25 ROWS ONLY ) AS [Limit4]
INNER JOIN [dbo].[ListItems] AS [Extent8] ON ([Limit4].[TenantId] = [Extent8].[TenantId]) AND ([Limit4].[BlarghId] = [Extent8].[BlarghId]) AND ([Limit4].[ListId] = [Extent8].[ListId])
WHERE (([Extent8].[Baz] > 0) OR ((LEN([Extent8].[Notes])) > 0) OR ((LEN([Extent8].[Value])) > 0)) AND ([Project7].[TenantId] = [Extent8].[TenantId]) AND ([Project7].[BlarghId] = [Extent8].[BlarghId]) AND ([Project7].[ListId] = [Extent8].[ListId]) AND (2 = [Extent8].[Baz])) AS [C5]
FROM ( SELECT
[Project5].[C1] AS [C1],
[Project5].[TenantId] AS [TenantId],
[Project5].[BlarghId] AS [BlarghId],
[Project5].[ListId] AS [ListId],
[Project5].[C2] AS [C2],
[Project5].[C3] AS [C3],
(SELECT
COUNT(1) AS [A1]
FROM (SELECT [Project6].[TenantId] AS [TenantId], [Project6].[BlarghId] AS [BlarghId], [Project6].[ListId] AS [ListId]
FROM ( SELECT
[Extent5].[TenantId] AS [TenantId],
[Extent5].[BlarghId] AS [BlarghId],
[Extent5].[ListId] AS [ListId],
[Extent5].[Created] AS [Created]
FROM [dbo].[Lists] AS [Extent5]
WHERE ([Extent5].[TenantId] = 8) AND ([Extent5].[BlarghId] = 8)
) AS [Project6]
ORDER BY [Project6].[Created] DESC
OFFSET 0 ROWS FETCH NEXT 25 ROWS ONLY ) AS [Limit3]
INNER JOIN [dbo].[ListItems] AS [Extent6] ON ([Limit3].[TenantId] = [Extent6].[TenantId]) AND ([Limit3].[BlarghId] = [Extent6].[BlarghId]) AND ([Limit3].[ListId] = [Extent6].[ListId])
WHERE (([Extent6].[Baz] > 0) OR ((LEN([Extent6].[Notes])) > 0) OR ((LEN([Extent6].[Value])) > 0)) AND ([Project5].[TenantId] = [Extent6].[TenantId]) AND ([Project5].[BlarghId] = [Extent6].[BlarghId]) AND ([Project5].[ListId] = [Extent6].[ListId]) AND (1 = [Extent6].[Baz])) AS [C4]
FROM ( SELECT
[Project3].[C1] AS [C1],
[Project3].[TenantId] AS [TenantId],
[Project3].[BlarghId] AS [BlarghId],
[Project3].[ListId] AS [ListId],
[Project3].[C2] AS [C2],
(SELECT
COUNT(1) AS [A1]
FROM (SELECT [Project4].[TenantId] AS [TenantId], [Project4].[BlarghId] AS [BlarghId], [Project4].[ListId] AS [ListId]
FROM ( SELECT
[Extent3].[TenantId] AS [TenantId],
[Extent3].[BlarghId] AS [BlarghId],
[Extent3].[ListId] AS [ListId],
[Extent3].[Created] AS [Created]
FROM [dbo].[Lists] AS [Extent3]
WHERE ([Extent3].[TenantId] = 8) AND ([Extent3].[BlarghId] = 8)
) AS [Project4]
ORDER BY [Project4].[Created] DESC
OFFSET 0 ROWS FETCH NEXT 25 ROWS ONLY ) AS [Limit2]
INNER JOIN [dbo].[ListItems] AS [Extent4] ON ([Limit2].[TenantId] = [Extent4].[TenantId]) AND ([Limit2].[BlarghId] = [Extent4].[BlarghId]) AND ([Limit2].[ListId] = [Extent4].[ListId])
WHERE (([Extent4].[Baz] > 0) OR ((LEN([Extent4].[Notes])) > 0) OR ((LEN([Extent4].[Value])) > 0)) AND ([Project3].[TenantId] = [Extent4].[TenantId]) AND ([Project3].[BlarghId] = [Extent4].[BlarghId]) AND ([Project3].[ListId] = [Extent4].[ListId]) AND ([Extent4].[Foo] = 1)) AS [C3]
FROM ( SELECT
[GroupBy1].[A1] AS [C1],
[GroupBy1].[K1] AS [TenantId],
[GroupBy1].[K2] AS [BlarghId],
[GroupBy1].[K3] AS [ListId],
[GroupBy1].[K4] AS [C2]
FROM ( SELECT
[Project2].[K1] AS [K1],
[Project2].[K2] AS [K2],
[Project2].[K3] AS [K3],
[Project2].[K4] AS [K4],
COUNT([Project2].[A1]) AS [A1]
FROM ( SELECT
[Project2].[TenantId] AS [K1],
[Project2].[BlarghId] AS [K2],
[Project2].[ListId] AS [K3],
1 AS [K4],
1 AS [A1]
FROM ( SELECT
[Extent2].[ListId] AS [ListId]
FROM (SELECT [Project1].[ListId] AS [ListId]
FROM ( SELECT
[Extent1].[ListId] AS [ListId],
[Extent1].[Created] AS [Created]
FROM [dbo].[Lists] AS [Extent1]
WHERE ([Extent1].[TenantId] = 8) AND ([Extent1].[BlarghId] = 8)
) AS [Project1]
ORDER BY [Project1].[Created] DESC
OFFSET 0 ROWS FETCH NEXT 25 ROWS ONLY ) AS [Limit1]
INNER JOIN [dbo].[ListItems] AS [Extent2] ON (([Limit1].[ListId] = [Extent2].[ListId])
WHERE ([Extent2].[Baz] > 0) OR ((LEN([Extent2].[Notes])) > 0) OR ((LEN([Extent2].[Value])) > 0)
) AS [Project2]
) AS [Project2]
GROUP BY [K1], [K2], [K3], [K4]
) AS [GroupBy1]
) AS [Project3]
) AS [Project5]
) AS [Project7]
) AS [Project9]
) AS [Project11]
) AS [Project13]
它根本不会将COUNT()语句组合在一起，而是将COUNT谓词移到单独的WHERE子句中。还要注意重复的分页子查询（使用OFFSET 0 ROW FETCH NEXT 25），而我的手写查询只执行一次。
答案 0 :(得分：1)
这是我的半解决方法：
我意识到最好的短期解决方案是在数据库中使用SQL（作为UDF FUNCTION或VIEW），这意味着某些数据代码因此必须在数据库（而不是将数据库用作“哑商店”）。
我首先创建了一个表值UDF，它接受一个表值参数，推理允许合成，代价是需要生成输入参数表（ListId值的数组来自分页查询）。但是，在这样做的过程中，我意识到Linq-to-Entities（版本6）还不支持Function Imports中的表值参数。
然后我推断一种更好的方法是将COUNT操作移动到VIEW（代表我手写查询中LEFT JOIN的一半），并且然后我可以让Linq加入现有的IQueryable，从而保留可组合性并生成有效的运行时查询（实际上，当我运行它时，查询需要34ms才能根据SQL Server Profiler执行，而旧的Linq生成的低效查询耗时830毫秒。
以下是我使用的内容：
CREATE VIEW ListItemStatistics AS
SELECT
ListId,
COUNT(*) AS [CountAll],
COUNT( CASE WHEN ... ) AS Count...
FROM
ListItems
WHERE
Foo = 'bar'
GROUP BY
ListId
然后从Linq内部：
IQueryable lists = GetListsQuery( 0, 25 );
var listsWithItemsStats = lists.
.Join(
inner: this.dbContext.ListItemStatistics,
outerKeySelector: list => list.ListId,
innerKeySelector: row => row.ListId,
resultSelector: (list,row) => new { list, row }
);
但是因为这确实使用了数据库端逻辑（在VIEW中），所以它并不理想。
答案 1 :(得分：1)
根据我的经验，当您在案例中使用应用过滤的聚合函数（例如Count(predicate）时，EF会生成此类查询。如果您使用条件求和（Count(condition)）替换Sum(condition ? 1 : 0)构造，您将获得更好的SQL查询：
var stats = db.ListItems
.Where(li => li.Qux != null)
.GroupBy(li => li.ListId)
.Select(grp => new
{
grp.Key,
Count = grp.Count(),
CountFooBar = grp.Sum(li => li.Foo == "bar" ? 1 : 0),
CountBaz5 = grp.Sum(li => li.Baz > 5 ? 1 : 0)
});
其他部分保持不变。在mod之前我得到了与发布的类似的SQL，这是我在这个小mod之后得到的：
SELECT
[Limit1].[ListId] AS [ListId],
[Limit1].[Name] AS [Name],
[Limit1].[DateCreated] AS [DateCreated],
[GroupBy1].[K1] AS [ListId1],
[GroupBy1].[A1] AS [C1],
[GroupBy1].[A2] AS [C2],
[GroupBy1].[A3] AS [C3]
FROM (SELECT [Extent1].[ListId] AS [ListId], [Extent1].[Name] AS [Name], [Extent1].[DateCreated] AS [DateCreated]
FROM [dbo].[List] AS [Extent1]
ORDER BY [Extent1].[DateCreated] ASC
OFFSET 0 ROWS FETCH NEXT 25 ROWS ONLY ) AS [Limit1]
INNER JOIN (SELECT
[Filter1].[K1] AS [K1],
COUNT([Filter1].[A1]) AS [A1],
SUM([Filter1].[A2]) AS [A2],
SUM([Filter1].[A3]) AS [A3]
FROM ( SELECT
[Extent2].[ListId] AS [K1],
1 AS [A1],
CASE WHEN (N'bar' = [Extent2].[Foo]) THEN 1 ELSE 0 END AS [A2],
CASE WHEN ([Extent2].[Baz] > 5) THEN 1 ELSE 0 END AS [A3]
FROM [dbo].[ListItem] AS [Extent2]
WHERE [Extent2].[Qux] IS NOT NULL
) AS [Filter1]
GROUP BY [K1] ) AS [GroupBy1] ON [Limit1].[ListId] = [GroupBy1].[K1]
UPDATE：以上只是手动SQL查询的LINQ等价物（当您调整连接代码以生成LEFT OUTER JOIN时）。但是，考虑到主数据分页，OUTER APPLY SQL查询可能会表现得更好。您可以让LINQ to Entities生成这样类型的查询：
var listsWithItemsStats = lists
.SelectMany(l => db.ListItems
.Where(li => li.ListId == l.ListId && li.Qux != null)
.GroupBy(li => li.ListId)
.Select(grp => new
{
grp.Key,
Count = grp.Count(),
CountFooBar = grp.Sum(li => li.Foo == "bar" ? 1 : 0),
CountBaz5 = grp.Sum(li => li.Baz > 5 ? 1 : 0)
})
.DefaultIfEmpty(),
(list, itemsStats) => new { list, itemsStats });