查詢運算式 (F#)
查詢運算式可以讓您查詢資料來源並將資料以一種所需格式。查詢運算式為 LINQ 提供支援 F#。
query { expression }
備註
查詢運算式是計算運算式的型別類似序列運算式。如同您透過提供程式碼指定序列在序列運算式，您可以提供程式碼指定一組資料在查詢運算式。在序列運算式中， yield 關鍵字可識別要傳回的資料為結果序列的一部分。在查詢運算式中， select 關鍵字執行相同的函式。除了 select 關鍵字以外，也支援 F# 非常類似 SQL SELECT 陳述式的部分的大量查詢運算子。這個簡單的查詢運算式的範例，其程式碼一起連接到 Northwind OData 來源。
// Use the OData type provider to create types that can be used to access the Northwind database.
// Add References to FSharp.Data.TypeProviders and System.Data.Services.Client
open Microsoft.FSharp.Data.TypeProviders
type Northwind = ODataService<"http://services.odata.org/Northwind/Northwind.svc">
let db = Northwind.GetDataContext()
// A query expression.
let query1 = query { for customer in db.Customers do
select customer }
query1
|> Seq.iter (fun customer -> printfn "Company: %s Contact: %s" customer.CompanyName customer.ContactName)
在上述程式碼範例中，查詢運算式在大括號。程式碼的意義在運算式為，則傳回每位客戶資料庫中的 Customers 資料表中的查詢結果。查詢運算式會傳回實作 IQueryable<T> 和 IEnumerable<T>的型別，和，以便重複使用 順序模組 ，在這個範例所示。
每個計算運算式型別從產生器類別中建立。查詢計算運算式產生器類別是 QueryBuilder。如需詳細資訊，請參閱計算運算式 (F#)與Linq.QueryBuilder 類別 (F#)。
查詢運算子
查詢運算子會傳回的記錄可讓您指定查詢的詳細資料，例如將 Standard 或指定結果排序順序。查詢來源必須支援查詢運算子。如果您嘗試使用不支援的查詢運算子， NotSupportedException 會擲回。
可以轉譯為 SQL 只提供的查詢運算式允許。例如，在中，當您使用 where 查詢運算子時，函式呼叫在運算式中不允許。
表 1 說明可用的查詢運算子。此外，請參閱 Table2 之後，比較 SQL 查詢和對等的 F# 查詢運算式本主題。某些查詢運算子不為陣列型別提供者支援。特別是， OData 型別提供者在查詢運算子會限制它支援由於提供者的限制。如需詳細資訊，請參閱 ODataService 型別提供者 (F#)。
此表會假設資料庫以下列形式:
範例資料庫圖表
.png "學生課程資料庫圖表")
程式碼也會顯示在下表中假設下列資料庫連接程式碼。專案應該加入至 System.Data、System.Data.Linq 和 FSharp.Data.TypeProviders 組件的參考。建立這個資料庫的程式碼是包含在這個主題的結尾。
open System
open Microsoft.FSharp.Data.TypeProviders
open System.Data.Linq.SqlClient
open System.Linq
open Microsoft.FSharp.Linq
type schema = SqlDataConnection<"Data Source=SERVER\INSTANCE;Initial Catalog=MyDatabase;Integrated Security=SSPI;">
let db = schema.GetDataContext()
// Needed for some query operator examples:
let data = [ 1; 5; 7; 11; 18; 21]
表 1。查詢運算子
Transact-SQL 和 F# 查詢運算式比較
下表顯示 F# 中一些常見的 Transact-SQL 查詢及其對等用法。程式碼在這個資料表中也假設相同設定型別的上一個資料表和相同初始程式碼提供者的資料庫。
表 2。Transact-SQL 和 F# 查詢運算式
下列程式碼可用來建立這些範例的範例資料庫。
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
USE [master];
GO
IF EXISTS (SELECT * FROM sys.databases WHERE name = 'MyDatabase')
DROP DATABASE MyDatabase;
GO
-- Create the MyDatabase database.
CREATE DATABASE MyDatabase COLLATE SQL_Latin1_General_CP1_CI_AS;
GO
-- Specify a simple recovery model
-- to keep the log growth to a minimum.
ALTER DATABASE MyDatabase
SET RECOVERY SIMPLE;
GO
USE MyDatabase;
GO
CREATE TABLE [dbo].[Course] (
[CourseID] INT NOT NULL,
[CourseName] NVARCHAR (50) NOT NULL,
PRIMARY KEY CLUSTERED ([CourseID] ASC)
);
CREATE TABLE [dbo].[Student] (
[StudentID] INT NOT NULL,
[Name] NVARCHAR (50) NOT NULL,
[Age] INT NULL,
PRIMARY KEY CLUSTERED ([StudentID] ASC)
);
CREATE TABLE [dbo].[CourseSelection] (
[ID] INT NOT NULL,
[StudentID] INT NOT NULL,
[CourseID] INT NOT NULL,
PRIMARY KEY CLUSTERED ([ID] ASC),
CONSTRAINT [FK_CourseSelection_ToTable] FOREIGN KEY ([StudentID]) REFERENCES [dbo].[Student] ([StudentID]) ON DELETE NO ACTION ON UPDATE NO ACTION,
CONSTRAINT [FK_CourseSelection_Course_1] FOREIGN KEY ([CourseID]) REFERENCES [dbo].[Course] ([CourseID]) ON DELETE NO ACTION ON UPDATE NO ACTION
);
CREATE TABLE [dbo].[LastStudent] (
[StudentID] INT NOT NULL,
[Name] NVARCHAR (50) NOT NULL,
[Age] INT NULL,
PRIMARY KEY CLUSTERED ([StudentID] ASC)
);
-- Insert data into the tables.
USE MyDatabase
INSERT INTO Course (CourseID, CourseName)
VALUES(1, 'Algebra I');
INSERT INTO Course (CourseID, CourseName)
VALUES(2, 'Trigonometry');
INSERT INTO Course (CourseID, CourseName)
VALUES(3, 'Algebra II');
INSERT INTO Course (CourseID, CourseName)
VALUES(4, 'History');
INSERT INTO Course (CourseID, CourseName)
VALUES(5, 'English');
INSERT INTO Course (CourseID, CourseName)
VALUES(6, 'French');
INSERT INTO Course (CourseID, CourseName)
VALUES(7, 'Chinese');
INSERT INTO Student (StudentID, Name, Age)
VALUES(1, 'Abercrombie, Kim', 10);
INSERT INTO Student (StudentID, Name, Age)
VALUES(2, 'Abolrous, Hazen', 14);
INSERT INTO Student (StudentID, Name, Age)
VALUES(3, 'Hance, Jim', 12);
INSERT INTO Student (StudentID, Name, Age)
VALUES(4, 'Adams, Terry', 12);
INSERT INTO Student (StudentID, Name, Age)
VALUES(5, 'Hansen, Claus', 11);
INSERT INTO Student (StudentID, Name, Age)
VALUES(6, 'Penor, Lori', 13);
INSERT INTO Student (StudentID, Name, Age)
VALUES(7, 'Perham, Tom', 12);
INSERT INTO Student (StudentID, Name, Age)
VALUES(8, 'Peng, Yun-Feng', NULL);
INSERT INTO CourseSelection (ID, StudentID, CourseID)
VALUES(1, 1, 2);
INSERT INTO CourseSelection (ID, StudentID, CourseID)
VALUES(2, 1, 3);
INSERT INTO CourseSelection (ID, StudentID, CourseID)
VALUES(3, 1, 5);
INSERT INTO CourseSelection (ID, StudentID, CourseID)
VALUES(4, 2, 2);
INSERT INTO CourseSelection (ID, StudentID, CourseID)
VALUES(5, 2, 5);
INSERT INTO CourseSelection (ID, StudentID, CourseID)
VALUES(6, 2, 6);
INSERT INTO CourseSelection (ID, StudentID, CourseID)
VALUES(7, 2, 3);
INSERT INTO CourseSelection (ID, StudentID, CourseID)
VALUES(8, 3, 2);
INSERT INTO CourseSelection (ID, StudentID, CourseID)
VALUES(9, 3, 1);
INSERT INTO CourseSelection (ID, StudentID, CourseID)
VALUES(10, 4, 2);
INSERT INTO CourseSelection (ID, StudentID, CourseID)
VALUES(11, 4, 5);
INSERT INTO CourseSelection (ID, StudentID, CourseID)
VALUES(12, 4, 2);
INSERT INTO CourseSelection (ID, StudentID, CourseID)
VALUES(13, 5, 3);
INSERT INTO CourseSelection (ID, StudentID, CourseID)
VALUES(14, 5, 2);
INSERT INTO CourseSelection (ID, StudentID, CourseID)
VALUES(15, 7, 3);
下列程式碼包含出現在本主題中的範例程式碼。
#if INTERACTIVE
#r "FSharp.Data.TypeProviders.dll"
#r "System.Data.dll"
#r "System.Data.Linq.dll"
#endif
open System
open Microsoft.FSharp.Data.TypeProviders
open System.Data.Linq.SqlClient
open System.Linq
[<Generate>]
type schema = SqlDataConnection<"Data Source=SERVER\INSTANCE;Initial Catalog=MyDatabase;Integrated Security=SSPI;">
let db = schema.GetDataContext()
let student = db.Student
let data = [1; 5; 7; 11; 18; 21]
open System
type Nullable<'T when 'T : ( new : unit -> 'T) and 'T : struct and 'T :> ValueType > with
member this.Print() =
if (this.HasValue) then this.Value.ToString()
else "NULL"
printfn "\ncontains query operator"
query {
for student in db.Student do
select student.Age.Value
contains 11
}
|> printfn "Is at least one student age 11? %b"
printfn "\ncount query operator"
query {
for student in db.Student do
select student
count
}
|> printfn "Number of students: %d"
printfn "\nlast query operator."
let num =
query {
for number in data do
sortBy number
last
}
printfn "Last number: %d" num
open Microsoft.FSharp.Linq
printfn "\nlastOrDefault query operator."
query {
for number in data do
sortBy number
lastOrDefault
}
|> printfn "lastOrDefault: %d"
printfn "\nexactlyOne query operator."
let student2 =
query {
for student in db.Student do
where (student.StudentID = 1)
select student
exactlyOne
}
printfn "Student with StudentID = 1 is %s" student2.Name
printfn "\nexactlyOneOrDefault query operator."
let student3 =
query {
for student in db.Student do
where (student.StudentID = 1)
select student
exactlyOneOrDefault
}
printfn "Student with StudentID = 1 is %s" student3.Name
printfn "\nheadOrDefault query operator."
let student4 =
query {
for student in db.Student do
select student
headOrDefault
}
printfn "head student is %s" student4.Name
printfn "\nselect query operator."
query {
for student in db.Student do
select student
}
|> Seq.iter (fun student -> printfn "StudentID, Name: %d %s" student.StudentID student.Name)
printfn "\nwhere query operator."
query {
for student in db.Student do
where (student.StudentID > 4)
select student
}
|> Seq.iter (fun student -> printfn "StudentID, Name: %d %s" student.StudentID student.Name)
printfn "\nminBy query operator."
let student5 =
query {
for student in db.Student do
minBy student.StudentID
}
printfn "\nmaxBy query operator."
let student6 =
query {
for student in db.Student do
maxBy student.StudentID
}
printfn "\ngroupBy query operator."
query {
for student in db.Student do
groupBy student.Age into g
select (g.Key, g.Count())
}
|> Seq.iter (fun (age, count) -> printfn "Age: %s Count at that age: %d" (age.Print()) count)
printfn "\nsortBy query operator."
query {
for student in db.Student do
sortBy student.Name
select student
}
|> Seq.iter (fun student -> printfn "StudentID, Name: %d %s" student.StudentID student.Name)
printfn "\nsortByDescending query operator."
query {
for student in db.Student do
sortByDescending student.Name
select student
}
|> Seq.iter (fun student -> printfn "StudentID, Name: %d %s" student.StudentID student.Name)
printfn "\nthenBy query operator."
query {
for student in db.Student do
where student.Age.HasValue
sortBy student.Age.Value
thenBy student.Name
select student
}
|> Seq.iter (fun student -> printfn "StudentID, Name: %d %s" student.Age.Value student.Name)
printfn "\nthenByDescending query operator."
query {
for student in db.Student do
where student.Age.HasValue
sortBy student.Age.Value
thenByDescending student.Name
select student
}
|> Seq.iter (fun student -> printfn "StudentID, Name: %d %s" student.Age.Value student.Name)
printfn "\ngroupValBy query operator."
query {
for student in db.Student do
groupValBy student.Name student.Age into g
select (g, g.Key, g.Count())
}
|> Seq.iter (fun (group, age, count) ->
printfn "Age: %s Count at that age: %d" (age.Print()) count
group |> Seq.iter (fun name -> printfn "Name: %s" name))
printfn "\n sumByNullable query operator"
query {
for student in db.Student do
sumByNullable student.Age
}
|> (fun sum -> printfn "Sum of ages: %s" (sum.Print()))
printfn "\n minByNullable"
query {
for student in db.Student do
minByNullable student.Age
}
|> (fun age -> printfn "Minimum age: %s" (age.Print()))
printfn "\n maxByNullable"
query {
for student in db.Student do
maxByNullable student.Age
}
|> (fun age -> printfn "Maximum age: %s" (age.Print()))
printfn "\n averageBy"
query {
for student in db.Student do
averageBy (float student.StudentID)
}
|> printfn "Average student ID: %f"
printfn "\n averageByNullable"
query {
for student in db.Student do
averageByNullable (Nullable.float student.Age)
}
|> (fun avg -> printfn "Average age: %s" (avg.Print()))
printfn "\n find query operator"
query {
for student in db.Student do
find (student.Name = "Abercrombie, Kim")
}
|> (fun student -> printfn "Found a match with StudentID = %d" student.StudentID)
printfn "\n all query operator"
query {
for student in db.Student do
all (SqlMethods.Like(student.Name, "%,%"))
}
|> printfn "Do all students have a comma in the name? %b"
printfn "\n head query operator"
query {
for student in db.Student do
head
}
|> (fun student -> printfn "Found the head student with StudentID = %d" student.StudentID)
printfn "\n nth query operator"
query {
for numbers in data do
nth 3
}
|> printfn "Third number is %d"
printfn "\n skip query operator"
query {
for student in db.Student do
skip 1
}
|> Seq.iter (fun student -> printfn "StudentID = %d" student.StudentID)
printfn "\n skipWhile query operator"
query {
for number in data do
skipWhile (number < 3)
select number
}
|> Seq.iter (fun number -> printfn "Number = %d" number)
printfn "\n sumBy query operator"
query {
for student in db.Student do
sumBy student.StudentID
}
|> printfn "Sum of student IDs: %d"
printfn "\n take query operator"
query {
for student in db.Student do
select student
take 2
}
|> Seq.iter (fun student -> printfn "StudentID = %d" student.StudentID)
printfn "\n takeWhile query operator"
query {
for number in data do
takeWhile (number < 10)
}
|> Seq.iter (fun number -> printfn "Number = %d" number)
printfn "\n sortByNullable query operator"
query {
for student in db.Student do
sortByNullable student.Age
select student
}
|> Seq.iter (fun student ->
printfn "StudentID, Name, Age: %d %s %s" student.StudentID student.Name (student.Age.Print()))
printfn "\n sortByNullableDescending query operator"
query {
for student in db.Student do
sortByNullableDescending student.Age
select student
}
|> Seq.iter (fun student ->
printfn "StudentID, Name, Age: %d %s %s" student.StudentID student.Name (student.Age.Print()))
printfn "\n thenByNullable query operator"
query {
for student in db.Student do
sortBy student.Name
thenByNullable student.Age
select student
}
|> Seq.iter (fun student ->
printfn "StudentID, Name, Age: %d %s %s" student.StudentID student.Name (student.Age.Print()))
printfn "\n thenByNullableDescending query operator"
query {
for student in db.Student do
sortBy student.Name
thenByNullableDescending student.Age
select student
}
|> Seq.iter (fun student ->
printfn "StudentID, Name, Age: %d %s %s" student.StudentID student.Name (student.Age.Print()))
printfn "All students: "
query {
for student in db.Student do
select student
}
|> Seq.iter (fun student -> printfn "%s %d %s" student.Name student.StudentID (student.Age.Print()))
printfn "\nCount of students: "
query {
for student in db.Student do
count
}
|> (fun count -> printfn "Student count: %d" count)
printfn "\nExists."
query {
for student in db.Student do
where (query { for courseSelection in db.CourseSelection do
exists (courseSelection.StudentID = student.StudentID) })
select student }
|> Seq.iter (fun student -> printfn "%A" student.Name)
printfn "\n Group by age and count"
query {
for n in db.Student do
groupBy n.Age into g
select (g.Key, g.Count())
}
|> Seq.iter (fun (age, count) -> printfn "%s %d" (age.Print()) count)
printfn "\n Group value by age."
query {
for n in db.Student do
groupValBy n.Age n.Age into g
select (g.Key, g.Count())
}
|> Seq.iter (fun (age, count) -> printfn "%s %d" (age.Print()) count)
printfn "\nGroup students by age where age > 10."
query {
for student in db.Student do
groupBy student.Age into g
where (g.Key.HasValue && g.Key.Value > 10)
select (g, g.Key)
}
|> Seq.iter (fun (students, age) ->
printfn "Age: %s" (age.Value.ToString())
students
|> Seq.iter (fun student -> printfn "%s" student.Name))
printfn "\nGroup students by age and print counts of number of students at each age with more than 1 student."
query {
for student in db.Student do
groupBy student.Age into group
where (group.Count() > 1)
select (group.Key, group.Count())
}
|> Seq.iter (fun (age, ageCount) ->
printfn "Age: %s Count: %d" (age.Print()) ageCount)
printfn "\nGroup students by age and sum ages."
query {
for student in db.Student do
groupBy student.Age into g
let total = query { for student in g do sumByNullable student.Age }
select (g.Key, g.Count(), total)
}
|> Seq.iter (fun (age, count, total) ->
printfn "Age: %d" (age.GetValueOrDefault())
printfn "Count: %d" count
printfn "Total years: %s" (total.ToString()))
printfn "\nGroup students by age and count number of students at each age, and display all with count > 1 in descending order of count."
query {
for student in db.Student do
groupBy student.Age into g
where (g.Count() > 1)
sortByDescending (g.Count())
select (g.Key, g.Count())
}
|> Seq.iter (fun (age, myCount) ->
printfn "Age: %s" (age.Print())
printfn "Count: %d" myCount)
printfn "\n Select students from a set of IDs"
let idList = [1; 2; 5; 10]
let idQuery = query { for id in idList do
select id }
query {
for student in db.Student do
where (idQuery.Contains(student.StudentID))
select student
}
|> Seq.iter (fun student ->
printfn "Name: %s" student.Name)
printfn "\nLook for students with Name match _e%% pattern and take first two."
query {
for student in db.Student do
where (SqlMethods.Like( student.Name, "_e%") )
select student
take 2
}
|> Seq.iter (fun student -> printfn "%s" student.Name)
printfn "\nLook for students with Name matching [abc]%% pattern."
query {
for student in db.Student do
where (SqlMethods.Like( student.Name, "[abc]%") )
select student
}
|> Seq.iter (fun student -> printfn "%s" student.Name)
printfn "\nLook for students with name matching [^abc]%% pattern."
query {
for student in db.Student do
where (SqlMethods.Like( student.Name, "[^abc]%") )
select student
}
|> Seq.iter (fun student -> printfn "%s" student.Name)
printfn "\nLook for students with name matching [^abc]%% pattern and select ID."
query {
for n in db.Student do
where (SqlMethods.Like( n.Name, "[^abc]%") )
select n.StudentID
}
|> Seq.iter (fun id -> printfn "%d" id)
printfn "\n Using Contains as a query filter."
query {
for student in db.Student do
where (student.Name.Contains("a"))
select student
}
|> Seq.iter (fun student -> printfn "%s" student.Name)
printfn "\nSearching for names from a list."
let names = [|"a";"b";"c"|]
query {
for student in db.Student do
if names.Contains (student.Name) then select student }
|> Seq.iter (fun student -> printfn "%s" student.Name)
printfn "\nJoin Student and CourseSelection tables."
query {
for student in db.Student do
join (for selection in db.CourseSelection ->
student.StudentID = selection.StudentID)
select (student, selection)
}
|> Seq.iter (fun (student, selection) -> printfn "%d %s %d" student.StudentID student.Name selection.CourseID)
printfn "\nLeft Join Student and CourseSelection tables."
query {
for student in db.Student do
leftOuterJoin (for selection in db.CourseSelection ->
student.StudentID = selection.StudentID) into result
for selection in result.DefaultIfEmpty() do
select (student, selection)
}
|> Seq.iter (fun (student, selection) ->
let selectionID, studentID, courseID =
match selection with
| null -> "NULL", "NULL", "NULL"
| sel -> (sel.ID.ToString(), sel.StudentID.ToString(), sel.CourseID.ToString())
printfn "%d %s %d %s %s %s" student.StudentID student.Name (student.Age.GetValueOrDefault()) selectionID studentID courseID)
printfn "\nJoin with count"
query {
for n in db.Student do
join (for e in db.CourseSelection -> n.StudentID = e.StudentID)
count
}
|> printfn "%d"
printfn "\n Join with distinct."
query {
for student in db.Student do
join (for selection in db.CourseSelection ->
student.StudentID = selection.StudentID)
distinct
}
|> Seq.iter (fun (student, selection) -> printfn "%s %d" student.Name selection.CourseID)
printfn "\n Join with distinct and count."
query {
for n in db.Student do
join (for e in db.CourseSelection -> n.StudentID = e.StudentID)
distinct
count
}
|> printfn "%d"
printfn "\n Selecting students with age between 10 and 15."
query {
for student in db.Student do
where (student.Age.Value >= 10 && student.Age.Value < 15)
select student
}
|> Seq.iter (fun student -> printfn "%s" student.Name)
printfn "\n Selecting students with age either 11 or 12."
query {
for student in db.Student do
where (student.Age.Value = 11 || student.Age.Value = 12)
select student
}
|> Seq.iter (fun student -> printfn "%s" student.Name)
printfn "\n Selecting students in a certain age range and sorting."
query {
for n in db.Student do
where (n.Age.Value = 12 || n.Age.Value = 13)
sortByNullableDescending n.Age
select n
}
|> Seq.iter (fun student -> printfn "%s %s" student.Name (student.Age.Print()))
printfn "\n Selecting students with certain ages, taking account of possibility of nulls."
query {
for student in db.Student do
where ((student.Age.HasValue && student.Age.Value = 11) ||
(student.Age.HasValue && student.Age.Value = 12))
sortByDescending student.Name
select student.Name
take 2
}
|> Seq.iter (fun name -> printfn "%s" name)
printfn "\n Union of two queries."
module Queries =
let query1 = query {
for n in db.Student do
select (n.Name, n.Age)
}
let query2 = query {
for n in db.LastStudent do
select (n.Name, n.Age)
}
query2.Union (query1)
|> Seq.iter (fun (name, age) -> printfn "%s %s" name (age.Print()))
printfn "\n Intersect of two queries."
module Queries2 =
let query1 = query {
for n in db.Student do
select (n.Name, n.Age)
}
let query2 = query {
for n in db.LastStudent do
select (n.Name, n.Age)
}
query1.Intersect(query2)
|> Seq.iter (fun (name, age) -> printfn "%s %s" name (age.Print()))
printfn "\n Using if statement to alter results for special value."
query {
for student in db.Student do
select (if student.Age.HasValue && student.Age.Value = -1 then
(student.StudentID, System.Nullable<int>(100), student.Age)
else (student.StudentID, student.Age, student.Age))
}
|> Seq.iter (fun (id, value, age) -> printfn "%d %s %s" id (value.Print()) (age.Print()))
printfn "\n Using if statement to alter results special values."
query {
for student in db.Student do
select (if student.Age.HasValue && student.Age.Value = -1 then
(student.StudentID, System.Nullable<int>(100), student.Age)
elif student.Age.HasValue && student.Age.Value = 0 then
(student.StudentID, System.Nullable<int>(100), student.Age)
else (student.StudentID, student.Age, student.Age))
}
|> Seq.iter (fun (id, value, age) -> printfn "%d %s %s" id (value.Print()) (age.Print()))
printfn "\n Multiple table select."
query {
for student in db.Student do
for course in db.Course do
select (student, course)
}
|> Seq.iteri (fun index (student, course) ->
if (index = 0) then printfn "StudentID Name Age CourseID CourseName"
printfn "%d %s %s %d %s" student.StudentID student.Name (student.Age.Print()) course.CourseID course.CourseName)
printfn "\nMultiple Joins"
query {
for student in db.Student do
join courseSelection in db.CourseSelection on
(student.StudentID = courseSelection.StudentID)
join course in db.Course on
(courseSelection.CourseID = course.CourseID)
select (student.Name, course.CourseName)
}
|> Seq.iter (fun (studentName, courseName) -> printfn "%s %s" studentName courseName)
printfn "\nMultiple Left Outer Joins"
query {
for student in db.Student do
leftOuterJoin (for courseSelection in db.CourseSelection ->
student.StudentID = courseSelection.StudentID) into g1
for courseSelection in g1.DefaultIfEmpty() do
leftOuterJoin (for course in db.Course ->
courseSelection.CourseID = course.CourseID) into g2
for course in g2.DefaultIfEmpty() do
select (student.Name, course.CourseName)
}
|> Seq.iter (fun (studentName, courseName) -> printfn "%s %s" studentName courseName)
然後這個完整輸出，當程式碼在 F# Interactive 中執行。