linq - 在同一查询中使用 join 和 group By 的 F# 查询表达式

Question

我决定学习如何在 F# 中使用查询表达式并找到了 the official Microsoft documentation .警告似乎没有详细说明，所以我确定我通过一个简单的修复遇到了问题，但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查询，同时执行连接和分组。例如，使用他们的示例“MyDatabase”，我想我会尝试找到每个学生注册的类(class)数量。编写以下查询后，编译器没有标记警告，但是当我去运行它时，它给出了错误.

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

我什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)

但这给出了更大的错误:

> 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，我并没有真正了解幕后发生的事情，不足以解决错误或引导自己找到解决方案。可以在这里查询我想要的吗？有解决方法吗？

Answer 1

当我尝试在本地运行它时，我得到了这个异常

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

我不确定为什么你会得到那个错误的不太漂亮的版本。但是，如果您将 GroupBy 从查询中拉出来，事情似乎可以正常工作。

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)