Question

我正在关注一个关于查询表达式的书中的示例。我没有完全按照本书中的内容输入，并且令人惊讶的是，即使对于F＃中的内连接查询，序列的顺序也很重要。这本书根本没有提到有关订单的任何内容，msdn只说订单很重要。但是为什么即使对于内部联接也很重要？

这不编译，它说namespace or module fa is not defined;同样适用于f：

query { for f in QuerySource.films do
        join fa in QuerySource.filmActors on (fa.FilmId = f.Id)
        join a in QuerySource.actors on (fa.ActorId = a.Id)
        select (f.Name, f.ReleaseYear, a.LastName, a.FirstName)
      }

但是这样做： query { for f in QuerySource.films do join fa in QuerySource.filmActors on (f.Id = fa.FilmId) join a in QuerySource.actors on (fa.ActorId = a.Id) select (f.Name, f.ReleaseYear, a.LastName, a.FirstName) }

EDIT1：不要认为这很重要，但为了澄清这里是QuerySource模块

module QuerySource = 
    open System

type Film = {Id : int; Name: string; ReleaseYear : int; Gross : Nullable<float>}
            override x.ToString() = sprintf "%s (%i)" x.Name x.ReleaseYear

type Actor = {Id : int; FirstName : string; LastName : string}
             override x.ToString() = sprintf "%s %s" x.LastName x.FirstName

type FilmActor = {FilmId : int; ActorId : int}

let films = 
        [   {Id = 1; Name = "The Terminator"; ReleaseYear = 1984; Gross = Nullable 38400000.0}
            {Id = 2; Name = "Predator"; ReleaseYear = 1987; Gross = Nullable 59735548.0}
            {Id = 3; Name = "Commando"; ReleaseYear = 1985; Gross = Nullable 38122105.0}
            {Id = 4; Name = "The Running Man"; ReleaseYear = 1987; Gross = Nullable<float>()}
            {Id = 5; Name = "Conan the Destroyer"; ReleaseYear = 1984; Gross = Nullable<float>()}
        ]
let actors = [{Id = 1; FirstName = "Arnold"; LastName = "Schwarzenegger"}
              {Id = 2; FirstName = "Linda"; LastName = "Hamilton"}
              {Id = 3; FirstName = "Carl"; LastName = "Weathers"}
              {Id = 4; FirstName = "Jesse"; LastName = "Ventura"}
              {Id = 5; FirstName = "Vernon"; LastName = "Wells"}
             ]
let filmActors = 
    [ {FilmId = 1; ActorId = 1}
      {FilmId = 1; ActorId = 2}
      {FilmId = 2; ActorId = 1}
      {FilmId = 2; ActorId = 3}
      {FilmId = 2; ActorId = 4}
      {FilmId = 3; ActorId = 1}
      {FilmId = 3; ActorId = 5}
      {FilmId = 4; ActorId = 1}
      {FilmId = 4; ActorId = 4} // internationally omitted for filmid = 5
    ]`

Answer 1

订单可能在性能方面具有重要意义。在连接表具有显着不同大小的情况下，查询将在开始第二个表之前开始在其中一个表中搜索所有对应值。如果它从较小的表开始，它会更快完成。

为什么内连接的顺序在F＃查询表达式中很重要

1 个答案: