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Wyrażenia kwerend (F#)

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Wyrażenia kwerendy umożliwiają kwerenda źródła danych i umieścić dane w żądanej formie.Wyrażenia kwerendy zapewniają obsługi zapytań LINQ w F#.

query { expression }

Uwagi

Wyrażenia kwerendy są rodzajem obliczeń wyrażenia podobnego do wyrażenia sekwencji.Podobnie, jak określa się sekwencji, podając kod w wyrażenie sekwencji, określonej zestaw danych poprzez podanie kodu w wyrażeniu kwerendy.W przypadku wyrażenia sekwencji yield słowa kluczowego identyfikuje dane zwracane jako część wynikowym sekwencji.W wyrażeniach kwerend select słowa kluczowego pełni tę samą funkcję.W uzupełnieniu do select słowa kluczowego, F# obsługuje również wiele operatorów zapytania, które są podobnie jak części instrukcji SQL SELECT.Oto przykład wyrażenia kwerendy prostej, wraz z kodem, który łączy ze źródłem Northwind OData.

// 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)

W poprzednim przykładzie wyrażenie kwerendy jest w nawiasy klamrowe.Znaczenie kodu w wyrażeniu jest, zwrotu każdy klient w tabeli Klienci w bazie danych w wynikach kwerendy.Wyrażenia kwerendy zwracają typ, który implementuje IQueryable i IEnumerable, a więc może się powtórzyć, za pomocą moduł Seq jak pokazano w przykładzie.

Każdy typ wyrażenia obliczeń jest zbudowany z klasy konstruktora.Klasa konstruktora dla wyrażenia obliczeń kwerendy jest QueryBuilder.Aby uzyskać więcej informacji, zobacz Wyrażenia obliczeń (F#) i Linq.QueryBuilder — Klasa (F#).

Operatory zapytań

Operatory kwerendy można określić dane, kwerendy, takie jak umieścić kryteria na rekordy mają być zwrócone lub określić kolejność sortowania wyników.Źródło kwerendy musi obsługiwać operator kwerendy.Jeśli nastąpi próba użycia kwerendy nieobsługiwany operator, NotSupportedException zostanie wyrzucony.

Tylko te wyrażenia, które mogą być tłumaczone SQL są dozwolone w wyrażeniach kwerend.Na przykład, żadne wywołania funkcji są dozwolone w wyrażeniach, korzystając z where operator kwerendy.

Tabela 1 zawiera operatory zapytań dostępne.Co więcej Zobacz tabela2, który porównuje zapytań SQL i równoważne F# kwerendy wyrażeń w dalszej części tego tematu.Niektórzy operatorzy kwerendy nie są obsługiwane przez niektórych dostawców typu.W szczególności dostawca typu OData jest ograniczony operatorów zapytania, które obsługuje ze względu na ograniczenia w OData.Aby uzyskać więcej informacji, zobacz ODataService typu dostawcy (F#).

Tej tabeli przyjęto założenie bazy danych w następującej formie:

Przykładowy Diagram bazy danych

Schemat bazy danych kurs dla studentów

Kod w tabelach, które należy wykonać zakłada też poniższy kod połączenia bazy danych.Projekty należy dodać odwołania do zestawów System.Data, System.Data.Linq i FSharp.Data.TypeProviders.Kod, który tworzy ta baza danych znajduje się na końcu tego tematu.

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]

W tabeli 1.Operatory zapytań

Operator

Opis

contains

Określa, czy wybrane elementy zawierają określonego elementu.

let isStudent11 =
    query {
        for student in db.Student do
        select student.Age.Value
        contains 11
    }

count

Zwraca liczbę zaznaczonych elementów.

let countOfStudents =
    query {
        for student in db.Student do
        select student
        count
    }

last

Zaznacza ostatni element tych stron do tej pory.

let number = 
    query {
        for number in data do
        last
    }

lastOrDefault

Zaznacza ostatni element do tej pory Możliwi lub wartość domyślną, jeśli element nie zostanie znaleziony.

let number =
    query {
        for number in data do
        where (number < 0)
        lastOrDefault
    }

exactlyOne

Powoduje zaznaczenie elementu pojedyncze, konkretne wybrane do tej pory.Jeśli występują wiele elementów, jest wyjątek.

let student =
    query {
        for student in db.Student do
        where (student.StudentID = 1)
        select student
        exactlyOne
    }

exactlyOneOrDefault

Powoduje zaznaczenie elementu pojedyncze, konkretne wybranych do tej pory lub wartość domyślną, jeśli nie można odnaleźć tego elementu.

let student =
    query {
        for student in db.Student do
        where (student.StudentID = 1)
        select student
        exactlyOneOrDefault
    }

headOrDefault

Zaznacza pierwszy element do tej pory Możliwi lub wartość domyślną, jeśli sekwencja zawiera żadnych elementów.

let student =
    query {
        for student in db.Student do
        select student
        headOrDefault
    }

select

Projekty, każdy z elementów wybranych do tej pory.

query {
    for student in db.Student do
    select student
    }

where

Wybiera elementy oparte na określonym predykatu.

query {
    for student in db.Student do
    where (student.StudentID > 4)
    select student
    }

minBy

Wybiera wartość dla każdego elementu, wybrany do tej pory i zwraca minimalną wartość wynikową.

let student =
    query {
        for student in db.Student do
        minBy student.StudentID
    }

maxBy

Wybiera wartość dla każdego elementu, wybrany do tej pory i zwraca maksymalną wartość wynikową.

let student =
    query {
        for student in db.Student do
        maxBy student.StudentID
    }

groupBy

Grupuje elementy do tej pory wybiera się zgodnie z określonym kluczem wyboru.

query {
    for student in db.Student do
    groupBy student.Age into g
    select (g.Key, g.Count())
    }

sortBy

Sortuje elementy zaznaczone do tej pory w kolejności rosnącej przez dany klucz sortowania.

query {
    for student in db.Student do
    sortBy student.Name
    select student
}

sortByDescending

Sortuje elementy zaznaczone do tej pory w porządku malejącym przez dany klucz sortowania.

query {
    for student in db.Student do
    sortByDescending student.Name
    select student
}

thenBy

Wykonuje kolejne kolejność elementów wybranych do tej pory w kolejności rosnącej przez dany klucz sortowania.Ten operator można używać tylko po sortBy, sortByDescending, thenBy, lub thenByDescending.

query {
    for student in db.Student do
    where student.Age.HasValue
    sortBy student.Age.Value
    thenBy student.Name
    select student
}

thenByDescending

Wykonuje kolejne kolejność elementów wybranych do tej pory w porządku malejącym przez dany klucz sortowania.Ten operator można używać tylko po sortBy, sortByDescending, thenBy, lub thenByDescending.

query {
    for student in db.Student do
    where student.Age.HasValue
    sortBy student.Age.Value
    thenByDescending student.Name
    select student
}

groupValBy

Wybiera wartość dla każdego elementu, wybrany do tej pory i grupuje elementy według podanego klucza.

query {
    for student in db.Student do
    groupValBy student.Name student.Age into g
    select (g, g.Key, g.Count())
    }

join

Jest całkowicie skorelowany dwa zestawy wybranych wartości oparta na dopasowanie klucze.Należy zauważyć, że zamówienie kluczy wokół = zarejestrować się w wyrażeniu sprzężenia jest znacząca.W sprzężeniu wszystkich, jeśli linia jest podzielona po -> symbol, wcięcie musi być wcięty co najmniej o ile to słowo kluczowe for.

query {
    for student in db.Student do 
    join selection in db.CourseSelection on
          (student.StudentID = selection.StudentID)
    select (student, selection)
}

groupJoin

Dwa zestawy wybranych wartości oparta na dopasowanie kluczy jest całkowicie skorelowany, a następnie grupuje wyniki.Należy zauważyć, że zamówienie kluczy wokół = zarejestrować się w wyrażeniu sprzężenia jest znacząca.

query {
    for student in db.Student do
    groupJoin courseSelection in db.CourseSelection on
               (student.StudentID = courseSelection.StudentID) into g
    for courseSelection in g do
    join course in db.Course on (courseSelection.CourseID = course.CourseID)
    select (student.Name, course.CourseName)
    }

leftOuterJoin

Dwa zestawy wybranych wartości oparta na dopasowanie kluczy jest całkowicie skorelowany, a następnie grupuje wyniki.Jeśli każda grupa jest pusta, grupy z wartością domyślną pojedynczego użyty.Należy zauważyć, że zamówienie kluczy wokół = zarejestrować się w wyrażeniu sprzężenia jest znacząca.

query {
    for student in db.Student do
    leftOuterJoin selection in db.CourseSelection on
                   (student.StudentID = selection.StudentID) into result
    for selection in result.DefaultIfEmpty() do
    select (student, selection)
    }

sumByNullable

Wybiera wartość dopuszczający wartość NULL dla każdego elementu, wybrany do tej pory i zwraca sumę tych wartości.Ewentualne dopuszczający wartość null nie ma wartość jest ignorowana.

query {
    for student in db.Student do
    sumByNullable student.Age
}

minByNullable

Wybiera wartość dopuszczający wartość NULL dla każdego elementu, wybrany do tej pory i zwraca wartość minimalną tych wartości.Ewentualne dopuszczający wartość null nie ma wartość jest ignorowana.

query {
    for student in db.Student do
    minByNullable student.Age
}

maxByNullable

Wybiera pustych wartości dla każdego elementu, wybrany do tej pory i zwraca maksymalną liczbę tych wartości.Ewentualne dopuszczający wartość null nie ma wartość jest ignorowana.

query {
    for student in db.Student do
    maxByNullable student.Age
    }

averageByNullable

Wybiera pustych wartości dla każdego elementu, wybrany do tej pory i zwraca średnią z tych wartości.Ewentualne dopuszczający wartość null nie ma wartość jest ignorowana.

query {
    for student in db.Student do
    averageByNullable (Nullable.float student.Age)
    }

averageBy

Wybiera wartość dla każdego elementu, wybrany do tej pory i zwraca średnią z tych wartości.

query {
    for student in db.Student do
    averageBy (float student.StudentID)
}

distinct

Wybiera elementy wyróżniające od elementów wybranych do tej pory.

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

exists

Określa, czy dowolny element wybrany do tej pory spełnia warunek.

query {
    for student in db.Student do
    where (query { for courseSelection in db.CourseSelection do
                   exists (courseSelection.StudentID = student.StudentID) })
    select student
}

find

Zaznacza pierwszy element wybrany do tej pory, który spełnia określone warunki.

query {
    for student in db.Student do
    find (student.Name = "Abercrombie, Kim")
}

all

Określa, czy wszystkie elementy zaznaczone do tej pory spełniają warunek.

query {
    for student in db.Student do
    all (SqlMethods.Like(student.Name, "%,%"))
}

head

Zaznacza pierwszy element z wybranych do tej pory.

query {
    for student in db.Student do
    head
}

nth

Powoduje zaznaczenie elementu określonym indeksem wśród tych wybranych do tej pory.

query {
    for numbers in data do
    nth 3
}

skip

Pomija określoną liczbę elementów wybranych do tej pory, a następnie wybiera pozostałych elementów.

query {
    for student in db.Student do
    skip 1
}

skipWhile

Omija elementy w sekwencji, tak długo, jak określony warunek jest spełniony, a następnie wybiera pozostałych elementów.

query {
    for number in data do
    skipWhile (number < 3)
    select student
    }

sumBy

Wybiera wartość dla każdego elementu, wybrany do tej pory i zwraca sumę tych wartości.

query {
   for student in db.Student do
   sumBy student.StudentID
   }

take

Wybiera określoną liczbę elementów sąsiadujących z zaznaczonymi do tej pory.

query {
   for student in db.Student do
   select student
   take 2
   }

takeWhile

Powoduje zaznaczenie elementów z sekwencji, tak długo, jak określony warunek jest prawdziwy, a następnie pomija pozostałe elementy.

query {
    for number in data do
    takeWhile (number < 10)
    }

sortByNullable

Sortuje elementy zaznaczone do tej pory w kolejności rosnącej przez dany klucz sortowania dopuszczający wartość null.

query {
    for student in db.Student do
    sortByNullable student.Age
    select student
}

sortByNullableDescending

Sortuje elementy zaznaczone do tej pory w porządku malejącym przez dany klucz sortowania dopuszczający wartość null.

query {
    for student in db.Student do
    sortByNullableDescending student.Age
    select student
}

thenByNullable

Wykonuje kolejne kolejność elementów wybranych do tej pory w kolejności rosnącej przez dany klucz sortowania dopuszczający wartość null.Ten operator może stanowić jedynie zaraz po sortBy, sortByDescending, thenBy, lub thenByDescending, lub ich odmian dopuszczający wartość null.

query {
    for student in db.Student do
    sortBy student.Name
    thenByNullable student.Age
    select student
}

thenByNullableDescending

Wykonuje kolejne kolejność elementów wybranych do tej pory w porządku malejącym przez dany klucz sortowania dopuszczający wartość null.Ten operator może stanowić jedynie zaraz po sortBy, sortByDescending, thenBy, lub thenByDescending, lub ich odmian dopuszczający wartość null.

query {
    for student in db.Student do
    sortBy student.Name
    thenByNullableDescending student.Age
    select student
}

Porównanie języka Transact-SQL i wyrażenia kwerendy F#

W poniższej tabeli przedstawiono niektóre typowe kwerendy języka Transact-SQL i ich odpowiedniki w F#.Kod w tej tabeli zakłada również tej samej bazy danych jako poprzedniej tabeli i tym samym kodem początkowym Aby skonfigurować dostawcę typu.

Tabela 2.Transact-SQL i wyrażenia kwerendy F#

Języka Transact-SQL (bez rozróżniania wielkości liter)

Język F# wyrażenie kwerendy (z uwzględnieniem wielkości liter)

Zaznacz wszystkie pola z tabeli.

SELECT * FROM Student
 
// All students.
query {
    for student in db.Student do
    select student
}

Zliczanie rekordów w tabeli.

SELECT COUNT(*) FROM Student
 
// Count of students.
query {
    for student in db.Student do        
    count
}

EXISTS

SELECT * FROM Student
WHERE EXISTS 
(SELECT * FROM CourseSelection
WHERE CourseSelection.StudentID = Student.StudentID)
 
// Find students who have signed up at least one course.
query {
    for student in db.Student do
    where (query { for courseSelection in db.CourseSelection do
                   exists (courseSelection.StudentID = student.StudentID) })
    select student
}

Grupowanie

SELECT Student.Age, COUNT(*) FROM Student
GROUP BY Student.Age
 
// Group by age and count.
query {
    for n in db.Student do
    groupBy n.Age into g
    select (g.Key, g.Count())
}
// OR
query {
    for n in db.Student do
    groupValBy n.Age n.Age into g
    select (g.Key, g.Count())
}

Grupowanie z warunkiem.

SELECT Student.Age, COUNT(*) 
FROM Student
GROUP BY Student.Age
HAVING student.Age > 10
 
// Group 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.Key, g.Count())
}

Grupowanie z warunkiem count.

SELECT Student.Age, COUNT(*)
FROM Student
GROUP BY Student.Age
HAVING COUNT(*) > 1
 
// Group students by age and count 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())
}

Grupowanie, Zliczanie i sumowanie.

SELECT Student.Age, COUNT(*), SUM(Student.Age) as total
FROM Student
GROUP BY Student.Age
 
// Group 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)
}

Grupowanie, Zliczanie i zamawiania przez licznik.

SELECT Student.Age, COUNT(*) as myCount
FROM Student
GROUP BY Student.Age
HAVING COUNT(*) > 1
ORDER BY COUNT(*) DESC
 
// Group students by age, 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())
}

INzestaw określonych wartości

SELECT *
FROM Student
WHERE Student.StudentID IN (1, 2, 5, 10)
 
// Select students where studentID is one of a given list.
let idQuery = query { for id in [1; 2; 5; 10] do
                      select id }
query { 
    for student in db.Student do
    where (idQuery.Contains(student.StudentID))
    select student
}

LIKE i TOP.

-- '_e%' matches strings where the second character is 'e'
SELECT TOP 2 * FROM Student
WHERE Student.Name LIKE '_e%'
 
// Look 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   
    }

LIKEz deseniem zgodna zestawem.

-- '[abc]%' matches strings where the first character is
-- 'a', 'b', 'c', 'A', 'B', or 'C'
SELECT * FROM Student
WHERE Student.Name LIKE '[abc]%'
 
open System.Data.Linq.SqlClient;

printfn "\nLook for students with Name matching [abc]%% pattern."
query {
    for student in db.Student do
    where (SqlMethods.Like( student.Name, "[abc]%") )
    select student  
    }

LIKEz zestawu któryś ze wzorców.

-- '[^abc]%' matches strings where the first character is
-- not 'a', 'b', 'c', 'A', 'B', or 'C'
SELECT * FROM Student
WHERE Student.Name LIKE '[^abc]%'
 
// Look for students with name matching [^abc]%% pattern.
query {
    for student in db.Student do
    where (SqlMethods.Like( student.Name, "[^abc]%") )
    select student  
    }

LIKEna jednym polu, ale wybierz inne pole.

SELECT StudentID AS ID FROM Student
WHERE Student.Name LIKE '[^abc]%'
 
open System.Data.Linq.SqlClient;

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)

LIKE, z wyszukiwanie podciągu.

SELECT * FROM Student
WHERE Student.Name like '%A%'
 
// Using Contains as a query filter.
query {
    for student in db.Student do
    where (student.Name.Contains("a"))
    select student
}

Proste JOIN z dwiema tabelami.

SELECT * FROM Student
JOIN CourseSelection 
ON Student.StudentID = CourseSelection.StudentID
 
// Join Student and CourseSelection tables.
query {
    for student in db.Student do 
    join selection in db.CourseSelection on
          (student.StudentID = selection.StudentID)
    select (student, selection)
}

LEFT JOINz dwóch tabel.

SELECT * FROM 
Student LEFT JOIN CourseSelection 
ON Student.StudentID = CourseSelection.StudentID
 
//Left Join Student and CourseSelection tables.
query {
    for student in db.Student do
    leftOuterJoin selection in db.CourseSelection on
                   (student.StudentID = selection.StudentID) into result
    for selection in result.DefaultIfEmpty() do
    select (student, selection)
    }

JOINzCOUNT

SELECT COUNT(*) FROM 
Student JOIN CourseSelection 
ON Student.StudentID = CourseSelection.StudentID
 
// Join with count.
query {
    for n in db.Student do 
    join e in db.CourseSelection on
          (n.StudentID = e.StudentID)
    count        
}

DISTINCT

SELECT DISTINCT StudentID FROM CourseSelection
 
// Join with distinct.
query {
    for student in db.Student do
    join selection in db.CourseSelection on
          (student.StudentID = selection.StudentID)
    distinct        
}

Zliczenie wyróżnionych.

SELECT DISTINCT COUNT(StudentID) FROM CourseSelection
 
// Join with distinct and count.
query {
    for n in db.Student do 
    join e in db.CourseSelection on
          n.StudentID = e.StudentID)
    distinct
    count       
}

BETWEEN

SELECT * FROM Student
WHERE Student.Age BETWEEN 10 AND 15
 
// Selecting students with ages between 10 and 15.
query {
    for student in db.Student do
    where (student.Age ?>= 10 && student.Age ?< 15)
    select student
}

OR

SELECT * FROM Student
WHERE Student.Age =11 OR Student.Age = 12
 
// Selecting students with age that's either 11 or 12.
query {
    for student in db.Student do
    where (student.Age.Value = 11 || student.Age.Value = 12)
    select student
}

ORz zamówieniem

SELECT * FROM Student
WHERE Student.Age =12 OR Student.Age = 13
ORDER BY Student.Age DESC
 
// 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
}

TOP, ORi zamówienia.

SELECT TOP 2 student.Name FROM Student
WHERE Student.Age = 11 OR Student.Age = 12
ORDER BY Student.Name DESC
 
// Selecting students with certain ages,
// taking account of the 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
    }

UNIONdwie kwerendy.

SELECT * FROM Student
UNION
SELECT * FROM lastStudent
 
// 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)

Przecięcie dwie kwerendy.

SELECT * FROM Student
INTERSECT
SELECT * FROM LastStudent
 
// 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)

CASEwarunek.

SELECT student.StudentID, 
    CASE Student.Age
        WHEN -1 THEN 100
        ELSE Student.Age
        END,
        Student.Age
from Student
 
// 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))
    }

Wiele przypadków.

SELECT Student.StudentID, 
    CASE Student.Age
        WHEN -1 THEN 100
        WHEN 0 THEN 1000
        ELSE Student.Age
    END,
    Student.Age
FROM Student
 
// Using if statement to alter results for 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>(1000), student.Age)
            else (student.StudentID, student.Age, student.Age))
    }

Wielu tabel.

SELECT * FROM Student, Course
 
// Multiple table select.
query {
        for student in db.Student do
        for course in db.Course do
        select (student, course)
}

Sprzężenia wielokrotne.

SELECT Student.Name, Course.CourseName
FROM Student
JOIN CourseSelection
ON CourseSelection.StudentID = Student.StudentID
JOIN Course
ON Course.CourseID = CourseSelection.CourseID
 
 // Multiple 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)
    }

Wiele Lewe sprzężenia zewnętrzne.

SELECT Student.Name, Course.CourseName
FROM Student
LEFT OUTER JOIN CourseSelection
ON CourseSelection.StudentID = Student.StudentID
LEFT OUTER JOIN Course
ON Course.CourseID = CourseSelection.CourseID
 
// Using leftOuterJoin with multiple joins.
query {
    for student in db.Student do
    leftOuterJoin courseSelection in db.CourseSelection on
        (student.StudentID = courseSelection.StudentID) into g1
    for courseSelection in g1.DefaultIfEmpty() do
    leftOuterJoin course in db.Course on
                   (courseSelection.CourseID = course.CourseID) into g2
    for course in g2.DefaultIfEmpty() do
    select (student.Name, course.CourseName)
    }

Poniższy kod może służyć do tworzenia przykładowej bazy danych dla tych przykładów.

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);

Poniższy kod zawiera przykładowy kod, który pojawia się w tym temacie.

#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)

A Oto pełne dane wyjściowe, po uruchomieniu tego kodu w F# interakcyjne.

                                                                                                                                    

Zobacz też

Informacje

Linq.QueryBuilder — Klasa (F#)

Inne zasoby

Materiały referencyjne dotyczące języka F#

Wyrażenia obliczeń (F#)