Linq-to-Entities：带有 WHERE 子句和投影的 LEFT OUTER JOIN

Question

我花了很长时间弄清楚如何将带有两个条件 where 子句的简单 SQL LEFT OUTER JOIN 转换为有效的 Linq-to-Entities 查询。只有两张桌子。我需要 Table1 中所有行的值，无论 Table2 中的匹配项如何，但 WHERE 子句使用 Table2 中的字段。在 SQL 中，这两个参数是 Table2WhereColumn1 和 Table2WhereColumn2，查询（有效）如下所示：

SELECT t1.Table1Id,
    t1.FieldDescription, 
    t2.FieldValue
FROM Table1 t1 WITH (NOLOCK)
LEFT JOIN Table2 t2 WITH (NOLOCK) ON t1.Table1Id = t2.Table1Id
WHERE (t2.Table2WhereColumn1 = @someId OR t2.Table2WhereColumn1 IS NULL)
AND (t2.Table2WhereColumn2 = @someOtherId OR t2.Table2WhereColumn2 IS NULL)
ORDER BY t1.OrderByColumn

我尝试过将Group Join 与DefaultIfEmpty() 一起使用，以及隐式连接（没有实际的Join 关键字），但我只获取表2 中具有值的项目的行。我确信这不会有帮助，但这是我一直在尝试但不起作用的 Linq 示例：

Public Shared Function GetProfilePreferencesForCedent(ByVal dc As EntityContext, _
                                                      ByVal where1 As Int32, _
                                                      ByVal where2 As Int32) _
                                                  As IQueryable(Of ProjectedEntity)
    Return From t1 In dc.Table1
           Group Join t2 In dc.Table2 _
                On t1.Table1Id Equals t2.Table1Id _
                Into t2g1 = Group _
           From t2gx In t2g1.DefaultIfEmpty(Nothing)
           Where (t2gx.Table2Where1 = where1 Or t2gx.Table2Where1 = Nothing) _
                And (t2gx.Table2Where2 = where2 Or t2gx.Table2Where2 = Nothing)
           Order By t1.SortOrder
           Select New ProjectedEntity With {
               .Table1Id = t1.Table1Id, _
               .FieldDescription = t1.FieldDescription, _
               .FieldValue = If(t2gx Is Nothing, String.Empty, t2gx.FieldValue) _
           }
End Function

Answer 1

试试这些查询，告诉我它们是否适合您。我还没有设置要测试的数据，但它们应该没问题。

请原谅我混合使用 C# 和 VB.NET。我曾经是一名 VB.NET 开发人员，但在过去的几年中，我主要使用 C# 工作，所以我现在在那里感觉更舒服了。

这是我为Table1 & Table2 创建的类：

public class Table1
{
    public int Table1Id { get; set; }
    public string FieldDescription { get; set; }
    public int OrderByColumn { get; set; }
}
public class Table2
{
    public int Table1Id { get; set; }
    public string FieldValue { get; set; }
    public int Table2WhereColumn1 { get; set; }
    public int Table2WhereColumn2 { get; set; }
}

现在 C# 中的查询应该是：

var query =
    from t1 in Table1
    join t2 in Table2 on t1.Table1Id equals t2.Table1Id into _Table2
    from _t2 in _Table2.DefaultIfEmpty()
    where _t2 == null ? true :
        _t2.Table2WhereColumn1 == @someId
        && _t2.Table2WhereColumn2 == @someOtherId
    orderby t1.OrderByColumn
    select new
    {
        t1.Table1Id,
        t1.FieldDescription,
        FieldValue = _t2 == null ? "" : _t2.FieldValue,
    };

并翻译成VB.NET：

Dim query = _
    From t1 In Table1 _
    Group Join t2 In Table2 On t1.Table1Id Equals t2.Table1Id Into _Table2 = Group _
    From _t2 In _Table2.DefaultIfEmpty() _
    Where If(_t2 Is Nothing, True, _t2.Table2WhereColumn1 = someId AndAlso  _
                                   _t2.Table2WhereColumn2 = someOtherId) _
    Order By t1.OrderByColumn _
    Select New With { _
            .Table1Id = t1.Table1Id, _
            .FieldDescription = t1.FieldDescription, _
            .FieldValue = If(_t2 Is Nothing, "", _t2.FieldValue) _
        }

让我知道它们是否有效。手指交叉。 :-)

Answer 2

就个人而言，如果存在左连接右侧的 where 条件，我通常更喜欢将它们放入连接条件中

在这种情况下，SQL 如下所示：

SELECT t1.Table1Id,
       t1.FieldDescription, 
       t2.FieldValue
FROM Table1 t1 WITH (NOLOCK)
LEFT JOIN Table2 t2 WITH (NOLOCK) ON t1.Table1Id = t2.Table1Id 
                                  AND t2.Table2WhereColumn1 = @someId
                                  AND t2.Table2WhereColumn2 = @someOtherId
ORDER BY t1.OrderByColumn

用于此的 LINQ 代码（在 C# 中）如下所示：

var query =
    from t1 in Table1
    join t2 in Table2 on new{a = t1.Table1Id, b = someId, c = someotherId} 
                         equals new {a = t2.Table1Id b = t2.Table2WhereColumn1, c = Table2WhereColumn2} 
    into _Table2
    from _t2 in _Table2.DefaultIfEmpty()
    orderby t1.OrderByColumn
    select new
    {
        t1.Table1Id,
        t1.FieldDescription,
        FieldValue = _t2 == null ? "" : _t2.FieldValue,
    };

未测试 - 但应该可以工作

Answer 3

我不会把这个答案归功于这个答案，但它很漂亮：LINQ to SQL - Left Outer Join with multiple join conditions

本质上，在子查询上使用扩展方法 where 子句，但必须在 DefaultIfEmpty() 之前使用它：

from p in context.Periods
join f in context.Facts on p.id equals f.periodid into fg
from fgi in fg.Where(f => f.otherid == 17).DefaultIfEmpty()
where p.companyid == 100
select f.value