Question

var result = (from t1 in _dbEntities.Charges
              join t2 in _dbEntities.ChargesTypes on t1.ChargesTypeID equals t2.ID
              where t1.StudentID == 1033
              select new {t2.Amount}).SingleOrDefault();

我想总结amount列如何做到这一点，在汇总之前应将其转换为int

Answer 1

首先，您使用SingleOrDefault，它只会选择一个值。您想要使用Sum。此外，您不需要匿名类型。你可以使用：

var result = (from t1 in _dbEntities.Charges
              join t2 in _dbEntities.ChargesTypes on t1.ChargesTypeID equals t2.ID
              where t1.StudentID == 1033
              select (int) t2.Ammount).Sum();

或者，您可以使用Sum的形式进行投影，可能使用不同形式的Join，因为您只需要t2值：

var result = _dbEntities.Charges
                        .Join(_dbEntities.ChargesTypes,
                              t1 => t1.ChargesTypeID,
                              t2 => t2.ID,
                              (t1, t2) => t2)
                        .Sum(t2 => (int) t2.Ammount);

甚至可以在联接中转换并在之后使用“普通”Sum：

var result = _dbEntities.Charges
                        .Join(_dbEntities.ChargesTypes,
                              t1 => t1.ChargesTypeID,
                              t2 => t2.ID,
                              (t1, t2) => (int) t2.Ammount)
                        .Sum();

编辑：如果数据库中的Ammount列（顺便说一下，Amount）是NVARCHAR，那么a）如果可能的话，更改它。如果数据库类型合适，生活总是更简单; b）如果你不能使用int.Parse：

var result = (from t1 in _dbEntities.Charges
              join t2 in _dbEntities.ChargesTypes on t1.ChargesTypeID equals t2.ID
              where t1.StudentID == 1033
              select int.Parse(t2.Ammount)).Sum();

如果这不起作用，可能需要在.NET端进行解析，例如。

var result = (from t1 in _dbEntities.Charges
              join t2 in _dbEntities.ChargesTypes on t1.ChargesTypeID equals t2.ID
              where t1.StudentID == 1033
              select t2.Ammount)
             .AsEnumerable() // Do the rest client-side
             .Sum(x => int.Parse(x));

Linq查询总结了数量列

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