Ø 前言
没有什么特别的，只是觉得 LINQ 的功能其实还是蛮强大的，所以简单记录下，算是工作笔记吧，有可能还能帮助到其他同学呢^_^。
Ø 下面主要使用了 C# 三元运算符实现实现 SQL 中的 CASE WHEN THEN 语句。
1) C#
const string deliverDM = "派送宣传册", haveKPInfo = "获得KP信息", talkWithKP = "和KP沟通", trial = "产品试样", turnover = "下单成交";
var query = (from t1 in DataContext.CustomerVisitInfo
where t1.SaleUserId == salesUserId && (t1.VisitTime >= mbdt && t1.VisitTime <= medt)
group t1 by t1.VisitStatus into g1
select new
{
VisitStatusName = (
g1.Key == (int)VisitStates.DeliverDM ? deliverDM
: g1.Key == (int)VisitStates.HaveKPInfo ? haveKPInfo
: g1.Key == (int)VisitStates.TalkWithKP ? talkWithKP
: g1.Key == (int)VisitStates.Trial ? trial
: g1.Key == (int)VisitStates.Turnover ? turnover : "其他"),
CustomerCount = g1.Count()
}).ToList();
2) 生成SQL：
exec sp_executesql N'SELECT
[GroupBy1].[K1] AS [VisitStatus],
CASE WHEN (1 = [GroupBy1].[K1]) THEN N''派送宣传册'' WHEN (2 = [GroupBy1].[K1]) THEN N''获得KP信息'' WHEN (3 = [GroupBy1].[K1]) THEN N''和KP沟通'' WHEN (4 = [GroupBy1].[K1]) THEN N''产品试样'' WHEN (5 = [GroupBy1].[K1]) THEN N''下单成交'' ELSE N''其他'' END AS [C1],
[GroupBy1].[A1] AS [C2]
FROM ( SELECT
[Extent1].[VisitStatus] AS [K1],
COUNT(1) AS [A1]
FROM [dbo].[CustomerVisitInfo] AS [Extent1]
WHERE ([Extent1].[SaleUserId] = @p__linq__0) AND ([Extent1].[VisitTime] >= @p__linq__1) AND ([Extent1].[VisitTime] <= @p__linq__2)
GROUP BY [Extent1].[VisitStatus]
) AS [GroupBy1]',N'@p__linq__0 bigint,@p__linq__1 datetime2(7),@p__linq__2 datetime2(7)',@p__linq__0=131,@p__linq__1='2017-05-01 00:00:00',@p__linq__2='2017-05-31 23:59:59'
Ø 思考：以上示例 THEN 中只是输出了常量字符串，也可以尝试输出其他语句结果，例如：嵌套子查询等。