Question

我决定学习如何在F＃中使用查询表达式并找到the official Microsoft documentation。这些警告似乎没有说清楚，所以我确定我已经通过一个简单的修复解决了问题，但我不知道为什么我会收到它显示的错误。

我的想法是编写一个同时执行连接和分组的查询。例如，使用他们的样本＆＃39; MyDatabase＆＃39;我想我会尝试找到每个学生注册的课程数。在编写以下查询后，编译器不会发出警告，但是当我去运行它时会出错。

我的查询表达式：

query {
    for student in db.Student do
    join course in db.CourseSelection
        on (student.StudentID = course.StudentID)

    groupBy student into group

    select (group.Key, group.Count())

}

|> Seq.iter (fun (student, classCount) -> printfn "Student %s has %i classes" student.Name classCount)

错误：

System.InvalidOperationException: Could not format node 'New' for execution as SQL.
   at System.Data.Linq.SqlClient.SqlFormatter.Visitor.VisitNew(SqlNew sox)
   at System.Data.Linq.SqlClient.SqlVisitor.Visit(SqlNode node)
>    at System.Data.Linq.SqlClient.SqlFormatter.Visitor.VisitAlias(SqlAlias alias)
   at System.Data.Linq.SqlClient.SqlVisitor.Visit(SqlNode node)
   at System.Data.Linq.SqlClient.SqlFormatter.Visitor.VisitSelect(SqlSelect ss)
   at System.Data.Linq.SqlClient.SqlVisitor.Visit(SqlNode node)
   at System.Data.Linq.SqlClient.SqlFormatter.Visitor.VisitScalarSubSelect(SqlSubSelect ss)
   at System.Data.Linq.SqlClient.SqlVisitor.Visit(SqlNode node)
   at System.Data.Linq.SqlClient.SqlFormatter.Visitor.VisitRow(SqlRow row)
   at System.Data.Linq.SqlClient.SqlFormatter.Visitor.VisitSelect(SqlSelect ss)
   at System.Data.Linq.SqlClient.SqlVisitor.Visit(SqlNode node)
   at System.Data.Linq.SqlClient.SqlFormatter.Visitor.Format(SqlNode node, Boolean isDebug)
   at System.Data.Linq.SqlClient.SqlFormatter.Format(SqlNode node)
   at System.Data.Linq.SqlClient.SqlProvider.BuildQuery(ResultShape resultShape, Type resultType, SqlNode node, ReadOnlyCollection`1 parentParameters, SqlNodeAnnotations annotations)
   at System.Data.Linq.SqlClient.SqlProvider.BuildQuery(Expression query, SqlNodeAnnotations annotations)
   at System.Data.Linq.SqlClient.SqlProvider.System.Data.Linq.Provider.IProvider.Execute(Expression query)
   at System.Data.Linq.DataQuery`1.System.Collections.Generic.IEnumerable<T>.GetEnumerator()
   at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()
   at Microsoft.FSharp.Collections.SeqModule.Iterate[T](FSharpFunc`2 action, IEnumerable`1 source)
   at <StartupCode$FSI_0008>.$FSI_0008.main@() in C:\Users\JDKS\Library\query expressions.fsx:line 129
Stopped due to error

我甚至认为我可以通过使用子查询来解决这个问题：

query {
    for student in db.Student do
    join course in db.CourseSelection
        on (student.StudentID = course.StudentID)

    let count = query {
        for s in student.Name do
        select course.CourseID
        count
    }

    select (student.Name, count)

}

|> Seq.iter (fun (student, classCount) -> printfn "Student %s has %i classes" student classCount)

但是这会产生更大的错误：

> System.NotSupportedException: Sequence operators not supported for type 'System.String'.
   at System.Data.Linq.SqlClient.SqlBinder.Visitor.ConvertToFetchedSequence(SqlNode node)
   at System.Data.Linq.SqlClient.SqlBinder.Visitor.VisitAlias(SqlAlias a)
   at System.Data.Linq.SqlClient.SqlVisitor.Visit(SqlNode node)
   at System.Data.Linq.SqlClient.SqlVisitor.VisitSource(SqlSource source)
   at System.Data.Linq.SqlClient.SqlBinder.Visitor.VisitSelect(SqlSelect select)
   at System.Data.Linq.SqlClient.SqlVisitor.Visit(SqlNode node)
   at System.Data.Linq.SqlClient.SqlBinder.Visitor.VisitAlias(SqlAlias a)
   at System.Data.Linq.SqlClient.SqlVisitor.Visit(SqlNode node)
   at System.Data.Linq.SqlClient.SqlVisitor.VisitSource(SqlSource source)
   at System.Data.Linq.SqlClient.SqlBinder.Visitor.VisitSelect(SqlSelect select)
   at System.Data.Linq.SqlClient.SqlVisitor.Visit(SqlNode node)
   at System.Data.Linq.SqlClient.SqlBinder.Visitor.VisitAlias(SqlAlias a)
   at System.Data.Linq.SqlClient.SqlVisitor.Visit(SqlNode node)
   at System.Data.Linq.SqlClient.SqlVisitor.VisitSource(SqlSource source)
   at System.Data.Linq.SqlClient.SqlBinder.Visitor.VisitSelect(SqlSelect select)
   at System.Data.Linq.SqlClient.SqlVisitor.Visit(SqlNode node)
   at System.Data.Linq.SqlClient.SqlVisitor.VisitSequence(SqlSelect sel)
   at System.Data.Linq.SqlClient.SqlVisitor.VisitScalarSubSelect(SqlSubSelect ss)
   at System.Data.Linq.SqlClient.SqlBinder.Visitor.VisitSubSelect(SqlSubSelect ss)
   at System.Data.Linq.SqlClient.SqlVisitor.Visit(SqlNode node)
   at System.Data.Linq.SqlClient.SqlBinder.Visitor.VisitExpression(SqlExpression expr)
   at System.Data.Linq.SqlClient.SqlBinder.Visitor.FetchExpression(SqlExpression expr)
   at System.Data.Linq.SqlClient.SqlBinder.Visitor.VisitNew(SqlNew sox)
   at System.Data.Linq.SqlClient.SqlVisitor.Visit(SqlNode node)
   at System.Data.Linq.SqlClient.SqlBinder.Visitor.VisitExpression(SqlExpression expr)
   at System.Data.Linq.SqlClient.SqlBinder.Visitor.VisitSelect(SqlSelect select)
   at System.Data.Linq.SqlClient.SqlVisitor.Visit(SqlNode node)
   at System.Data.Linq.SqlClient.SqlBinder.Visitor.VisitAlias(SqlAlias a)
   at System.Data.Linq.SqlClient.SqlVisitor.Visit(SqlNode node)
   at System.Data.Linq.SqlClient.SqlVisitor.VisitSource(SqlSource source)
   at System.Data.Linq.SqlClient.SqlBinder.Visitor.VisitSelect(SqlSelect select)
   at System.Data.Linq.SqlClient.SqlVisitor.Visit(SqlNode node)
   at System.Data.Linq.SqlClient.SqlBinder.Visitor.VisitIncludeScope(SqlIncludeScope scope)
   at System.Data.Linq.SqlClient.SqlVisitor.Visit(SqlNode node)
   at System.Data.Linq.SqlClient.SqlBinder.Bind(SqlNode node)
   at System.Data.Linq.SqlClient.SqlProvider.BuildQuery(ResultShape resultShape, Type resultType, SqlNode node, ReadOnlyCollection`1 parentParameters, SqlNodeAnnotations annotations)
   at System.Data.Linq.SqlClient.SqlProvider.BuildQuery(Expression query, SqlNodeAnnotations annotations)
   at System.Data.Linq.SqlClient.SqlProvider.System.Data.Linq.Provider.IProvider.Execute(Expression query)
   at System.Data.Linq.DataQuery`1.System.Collections.Generic.IEnumerable<T>.GetEnumerator()
   at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()
   at Microsoft.FSharp.Collections.SeqModule.Iterate[T](FSharpFunc`2 action, IEnumerable`1 source)
   at <StartupCode$FSI_0015>.$FSI_0015.main@() in C:\Users\JDKS\Library\query expressions.fsx:line 144
Stopped due to error

我的猜测是，我不能真正了解幕后发生的事情，足以解决错误或引导自己找到解决方案。有可能在这里查询我想要的东西吗？有解决方法吗？

Answer 1

当我尝试在本地运行时，我遇到了这个异常

System.Exception: Grouping over multiple tables is not supported yet

我不确定为什么你会得到那个不太漂亮的错误版本。但是如果你将GroupBy从查询中拉出来，事情似乎就可以了。

query {
    for student in db.Student do
    join course in db.CourseSelection
        on (student.StudentID = course.StudentID)
    select (student.Name, course.Id)
} 
|> Seq.countBy snd
|> Seq.iter (fun (student, classCount) -> printfn "Student %s has %i classes" student classCount)

Answer 2

我认为您的第一个查询无效的原因是groupBy student into group。

我认为你真正想写的是groupBy student.StudentID into group。

Answer 3

我从similar question找到答案：

query {
    for student in db.Student do

    join course in db.CourseSelection
        on (student.StudentID = course.StudentID)

    let count = query { 
        for c in db.CourseSelection do
        where (c.StudentID = student.StudentID)
        select c
        count
    }


    select (student.Name, count)
    distinct

}

|> Seq.iter (fun (student, classCount) -> printfn "Student %s has %i classes" student classCount)

这些查询表达式的基础范例仍然无法实现。

编辑：

我发现您可以隐式加入而无需使用join运算符

query {
    for student in db.Student do
    let count = query {
        for course in student.CourseSelection do
        count    
    }

    select (student.Name, count)
}
|> Seq.iter (fun (name, count) -> printfn "%s has %i courses" name count)

F＃使用join＆amp; amp;查询表达式groupBy在同一个查询中

3 个答案: