Entity Framework 分组查询翻译成多个 SELECT

Question

有人可以帮忙告诉我为什么这个 LINQ 查询在 EF 中被翻译成多个 SELECT 吗？

var query = db.ReportedFulfillments.GroupBy(x => x.ContractId).Select(grouping => new
    {
         ContractId = grouping.Key,
         Total = grouping.Count(),
         FU = grouping.Count(x => x.Month == 1 && x.Value == 1),
         BR = grouping.Count(x => x.Month == 1 && x.Value == 2)
    }

我以为 EF 会输出这样的东西：

SELECT ContractId,
 Count(*) AS Total,
 COUNT(CASE WHEN [Month] = 1 AND [Value] = 1 THEN Value END) AS FU,
 COUNT(CASE WHEN [Month] = 1 AND [Value] = 2 THEN Value END) AS BR,
FROM ReportedFulfillments GROUP BY ContractId

我正在使用 EntityFramework 6.2.0

Answer 1

不久，GroupBy 分组结果集的 LINQ 条件 Count 没有得到很好的支持（您期望的 SQL 依赖于 SQL COUNT(expr)它排除了 NULL 并且没有 LINQ 等价物）。

但是等效的条件 Sum 得到了很好的支持和翻译，所以不用

FU = grouping.Count(x => x.Month == 1 && x.Value == 1),
BR = grouping.Count(x => x.Month == 1 && x.Value == 2)

使用

FU = grouping.Sum(x => x.Month == 1 && x.Value == 1 ? 1 : 0),
BR = grouping.Sum(x => x.Month == 1 && x.Value == 2 ? 1 : 0)

Entity Framework 分组查询翻译成多个 SELECT

Entity Framework grouped query translated into multiple SELECTs

c#

linq

entity-framework

entity-framework-6