c# - linq sql查找重复项但将null和空字符串视为相同

Question

我有半百万条记录表，我需要找到重复项。所以我使用我创建的这段代码:

var dups2 = from m in mg_B
    group m by new { m.Addr1, m.Addr2, m.City, m.State }
    into g
    where g.Count() > 1
    select g;

此代码的问题在于，它不会将 addr1 为空字符串“”且分别为 NULL 的 2 条记录视为重复项。

基本上，在比较字段的空值和空值时，它认为它们不同，但我需要被视为相同。

我知道我可以遍历每条记录并将空值替换为“”，但我花了计算机 1 分钟来遍历 4000 条记录。当有人点击按钮时，这将重复完成。

我发现这个 null 空字符串问题是因为我最初创建了一个只有一些字段的类(该表有 40 多个字段)。

List<CombineClass> mg = (from m in db.MG_Backup
   where m.IsArchived == false
   select new CombineClass { id = m.ID, name = m.Name, addr1 = string.IsNullOrEmpty(m.Addr1) ? "" : m.Addr1, addr2 = string.IsNullOrEmpty(m.Addr2) ? "" : m.Addr2, city = m.City, state = m.State }).ToList(); 

有什么想法吗？

Answer 1

此版本与 Linq-to-Sql/Linq-to-Entities 兼容

var dups2 = from m in mg_B
    group m by new 
    { 
        Addr1 = m.Addr1 ?? string.Empty, 
        Addr2 = m.Addr2 ?? string.Empty, 
        City  = m.City ?? string.Empty, 
        State = m.State ?? string.Empty,
    }
    into g
    where g.Count() > 1
    select g;

生成的sql看起来有点像这样:

-- Parameters
DECLARE @p0 NVarChar(1000) = ''
DECLARE @p1 NVarChar(1000) = ''
DECLARE @p2 NVarChar(1000) = ''
DECLARE @p3 NVarChar(1000) = ''
DECLARE @p4 Int = 1

SELECT [t2].[value2] AS [Addr1], [t2].[value22] AS [Addr2], [t2].[value3] AS [City], [t2].[value3] AS [State]
FROM (
    SELECT COUNT(*) AS [value], [t1].[value] AS [value2], [t1].[value2] AS [value22], [t1].[value3], [t1].[value4]
    FROM (
        SELECT COALESCE([t0].[Addr1],@p0) AS [value], COALESCE([t0].[Addr2],@p1) AS [value2], COALESCE([t0].[City],@p2) AS [value3], COALESCE([t0].[State],@p3) AS [value4]
        FROM [SettingSystemNodes] AS [t0]
        ) AS [t1]
    GROUP BY [t1].[value], [t1].[value2], [t1].[value3], [t1].[value4]
    ) AS [t2]
WHERE [t2].[value] > @p4

请注意，如果您将 string.Empty 设置为查询之前的局部变量，甚至是查询中的 let 变量，则只有一个参数将用于空字符串.