如何在 LINQ 中正确执行 LEFT JOIN？

Question

这是我的查询。我想在 LINQ

中执行此操作

SELECT tc.AppId, 
   tc.ConfigCode, 
   tc.ConfigId, 
   tc.ConfigType, 
   COALESCE(tm.ConfigValue, tc.ConfigValue) AS ConfigValue, 
    CASE 
      WHEN tc.ConfigType = 'Application' 
         THEN tc.AppId
      ELSE tm.Id
    END as Id,
FROM   dbo.Configs tc 
   LEFT JOIN dbo.ConfigValues tm 
          ON tc.ConfigId = tm.ConfigId

这是我的查询，但我没有得到正确的结果

db.Configs
.Join(db.ConfigValues.DefaultIfEmpty(), tc => tc.ConfigId, tm => tm.ConfigId, (tc, tm) => new { tc = tc, tm = tm })
.Select(r => new {
    AppId = (Guid?)r.tc.AppId,
    ConfigCode = r.tc.ConfigCode,
    ConfigId = r.tc.ConfigId,
    ConfigValue = (r.tm.ConfigValue ?? r.tc.ConfigValue),
    Id = (Guid)(r.tc.ConfigType == "Application" ? r.tc.AppId : r.tm.Id),
});

Answer 1

我认为您只是缺少 DefaultIfEmpty()。这 '？'在 r.tm?.ConfigValue??... 捕获 'r.tm'

的 NullPointer

db.Configs
.Join(db.ConfigValues.DefaultIfEmpty(), tc => tc.ConfigId, tm => tm.ConfigId, (tc, tm) => new { tc = tc, tm = tm })
.Select(r => new {
    AppId = (Guid?)r.tc.AppId,
    ConfigCode = r.tc.ConfigCode,
    ConfigId = r.tc.ConfigId,
    ConfigValue = (r.tm.ConfigValue ?? r.tc.ConfigValue),
    Id = (Guid)(r.tc.ConfigType == "Application" ? r.tc.AppId : r.tm.Id)
}).DefaultIfEmpty(AppId = (Guid?)r.tc.AppId,
                  ConfigCode = r.tc.ConfigCode,
                  ConfigId = r.tc.ConfigId,
                  ConfigValue = (r.tm?.ConfigValue ?? r.tc.ConfigValue),
                  Id = (Guid)(r.tc.ConfigType == "Application" ? r.tc.AppId : r.tm.Id));

Answer 2

您的查询可能类似于


db.Configs
.LeftJoin(db.ConfigValues, tc => tc.ConfigId, tm => tm.ConfigId)
.Select(r => new {tc = r.Left, tm = r.Right})
.Select(r => new {
    AppId = (Guid?)r.tc.AppId,
    ConfigCode = r.tc.ConfigCode,
    ConfigId = r.tc.ConfigId,
    ConfigValue = (r.tm.ConfigValue ?? r.tc.ConfigValue),
    Id = (Guid)(r.tc.ConfigType == "Application" ? r.tc.AppId : r.tm.Id),
});

如果您使用以下扩展方法。这将使您的代码在使用流畅版本的 LINQ 时更具可读性，并且可重用，因为您可以在其他 LINQ 查询中使用 LeftJoin/RightJoin。


public class LeftJoinResult
{
    public TLeft Left { get; set; }
    public TRight Right { get; set; }
}

public class RightJoinResult
{
    public TRight Right { get; set; }
    public TLeft Left { get; set; }
}

public static IQueryable> LeftJoin(this IQueryable left, IQueryable right, Expression> leftKeySelector, Expression> rightKeySelector)
    where TRight : class
{
    return left.GroupJoin(right, leftKeySelector, rightKeySelector, (l, r) => new
    {
        Left = l,
        Matches = r.DefaultIfEmpty(),
    })
    .SelectMany(j => j.Matches, (j, m) => new LeftJoinResult
    {
        Left = j.Left,
        Right = m,
    });
}

public static IQueryable> RightJoin(this IQueryable right, IQueryable left, Expression> rightKeySelector, Expression> leftKeySelector)
    where TLeft : class
{
    return right.GroupJoin(left, rightKeySelector, leftKeySelector, (r, l) => new
    {
        Right = r,
        Matches = l.DefaultIfEmpty(),
    })
    .SelectMany(j => j.Matches, (j, m) => new RightJoinResult
    {
        Right = j.Right,
        Left = m,
    });
}

如何在 LINQ 中正确执行 LEFT JOIN？

How to do LEFT JOIN in LINQ correctly?

c#

linq

linq-to-entities

entity-framework