linq - Linq to Sql IsNull 用现有 FK 替换 NULL FK

Question

我需要将 sql 查询转换为 Linq to SQL 查询，但我无法得到我期望的结果：我有 2 个表，Base，M 表

BaseOne
    {
    MID (FK) , points to M
    MChildID(FK, Nullable), points to M
    }

    M {
    ID(PK)
    }

简化选择：

SELECT * from BaseOne as f 
LEFT JOIN M as m ON ISNULL(f.MChildID, f.MID) = m.ID

我试过这个：

from f in BaseOne 
from m in M.Where(function(x) If(f.MChildID.HasValue,x.ID.Equals(f.MChildID.Value),x.ID.Equals(f.MID.Value))).DefaultIfEmpty

它生成了这个 SQL：

...
        (CASE 
            WHEN [t0].[MID] IS NOT NULL THEN 
                (CASE 
                    WHEN [t1].[ID] = ([t0].[MID]) THEN 1
                    WHEN NOT ([t1].[ID] = ([t0].[MID])) THEN 0
                    ELSE NULL
                 END)
            ELSE 
                (CASE 
                    WHEN [t1].[ID] = ([t0].[MChildID]) THEN 1
                    WHEN NOT ([t1].[ID] = ([t0].[MChildID])) THEN 0
                    ELSE NULL
                 END)
         END)) = 1
    LEFT OUTER JOIN ...

我遵循了这些说明，但这个例子并不是我尝试做的。
有什么建议么？

Answer 1

您是否尝试过 VB 的二进制if() 运算符？

from baseOne in BaseOne _
join m in M on if(baseOne.MChildID,baseOne.MID) equals m.ID _
select baseOne,m

它产生一个COALESCE，类似于 ISNULL。

FROM [BaseOne] AS [t0]
INNER JOIN [M] AS [t1] ON (COALESCE([t0].[MChildID],[t0].[MID])) = [t1].[ID]