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Výrazy dotazu (F#)

  • Článek

Po zadání výrazu umožňují dotaz na zdroj dat a vložit data v požadované formě.Dotaz výrazy podporují LINQ v F#.

query { expression }

Poznámky

Dotaz výrazy jsou typem výpočtu výraz podobný sekvence výrazů.Stejně jako posloupnost určíte zadáním kódu v výraz sekvence, určit sadu dat zadáním kódu ve výrazu dotazu.V výraz sekvence yield klíčové slovo označuje data mají být vráceny jako součást výsledné sekvence.Ve výrazech dotazu select klíčové slovo plní stejnou funkci.Navíc select klíčové slovo, F# také podporuje řadu operátorů pro dotazování, které jsou, podobně jako část příkazu SQL SELECT.Zde je příklad jednoduchého dotazu výraz, spolu s kódem, který spojí se zdrojem 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)

V předchozím příkladu kódu výrazu dotazu je do složených závorek.Význam kódu ve výrazu, vrátit každého zákazníka v tabulce Zákazníci v databázi ve výsledcích dotazu.Po zadání výrazu vrátí typ, který implementuje IQueryable<T> a IEnumerable<T>, a tak mohou být vstupní pomocí Seq modul jako příklad ukazuje.

Každý typ výpočtu výraz je sestaven z třídy tvůrce.Třída tvůrce pro výpočet výrazu dotazu je QueryBuilder.Další informace naleznete v tématu Výpočet výrazy (F#) a Třída LINQ.QueryBuilder (F#).

Operátory dotazů

Operátory dotazů umožňují určit podrobnosti dotazu, jako například umístění kritéria na záznamy, které mají být vráceny nebo určit pořadí řazení výsledků.Dotaz na zdroj musí podporovat operátor dotazu.Pokud se pokusíte použít operátor dotazu není podporován, NotSupportedException bude vyvolána výjimka.

Ve výrazech dotazu jsou povoleny pouze výrazy, které mohou být převedeny na SQL.Například žádná volání funkce jsou povoleny v výrazy při použití where operátor dotazu.

V tabulce 1 uvedeny operátory dotazů k dispozici.Více informací naleznete tabulka2, který porovnává dotazy SQL a ekvivalentní F# dotaz výrazy dále v tomto tématu.Někteří poskytovatelé typ nepodporuje některé operátory dotazů.Typ zprostředkovatele OData je zejména omezené operátory pro dotazování, které podporují z důvodu omezení v OData.Další informace naleznete v tématu ODataService typ zprostředkovatele (F#).

V této tabulce se předpokládá databáze v následujícím tvaru:

Ukázka diagramu databáze

Student kurzu databázového diagramu

Kód v tabulkách, které následují také předpokládá následující kód pro připojení databáze.Projekty by měly přidat odkazy na sestavení System.Data System.Data.Linq a FSharp.Data.TypeProviders.Je kód, který vytváří tuto databázi naleznete na konci tohoto tématu.

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]

Tabulka 1.Operátory dotazů

Operátor

Description

contains

Určuje, zda vybrané prvky obsahují zadaný element.

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

count

Vrátí počet vybraných elementů.

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

last

Vybere poslední prvek ti tak daleko.

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

lastOrDefault

Vybere poslední prvek ti doposud nebo výchozí hodnotu, pokud je nalezen žádný element.

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

exactlyOne

Vybere jeden, určitý prvek vybrali tak daleko.Pokud existují více prvků, je vyvolána výjimka.

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

exactlyOneOrDefault

Vybere jeden, určitý prvek ti doposud nebo výchozí hodnotu, pokud tento prvek nebyl nalezen.

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

headOrDefault

Vybere první prvek ti doposud nebo výchozí hodnotu v případě, že pořadí neobsahuje žádné prvky.

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

select

Projekty jednotlivých prvků, které jsou vybrány tak daleko.

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

where

Vybere elementy, které jsou založeny na zadaný predikátu.

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

minBy

Vybere hodnoty pro každý prvek dosud vybrán a vrátí minimální výsledná hodnota.

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

maxBy

Vybere hodnoty pro každý prvek dosud vybrán a vrátí nejvyšší výslednou hodnotu.

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

groupBy

Skupiny prvků zatím vybraný podle zadaného klíče selektor.

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

sortBy

Seřadí prvky zvolila doposud ve vzestupném pořadí dané třídicím klíčem.

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

sortByDescending

Seřadí prvky zatím vybrán v sestupném pořadí podle daného klíče řazení.

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

thenBy

Provádí následné pořadí prvků vybrané doposud ve vzestupném pořadí podle daného klíče řazení.Tento operátor lze použít pouze po sortBy, sortByDescending, thenBy, nebo thenByDescending.

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

thenByDescending

Provádí následné pořadí prvků zatím vybrán v sestupném pořadí podle daného klíče řazení.Tento operátor lze použít pouze po sortBy, sortByDescending, thenBy, nebo thenByDescending.

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

groupValBy

Vybere hodnoty pro každý prvek dosud vybrán a skupiny prvků v daném klíči.

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

join

Koreluje dvě sady hodnot vybraných na základě shody klíčů.Všimněte si, že pořadí klíče kolem = znak ve výrazu join je významný.Ve všech spojení, pokud po je rozdělit řádek -> symbol, odsazení musí být odsazené alespoň pokud jde o klíčové slovo for.

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

groupJoin

Koreluje dvě sady hodnot vybraných na základě shody klíčů a skupiny výsledků.Všimněte si, že pořadí klíče kolem = znak ve výrazu join je významný.

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

Koreluje dvě sady hodnot vybraných na základě shody klíčů a skupiny výsledků.Pokud žádné skupiny je prázdný, je použita skupiny s jedinou výchozí hodnotu.Všimněte si, že pořadí klíče kolem = znak ve výrazu join je významný.

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

Vybere s možnou NULL hodnotou pro každý prvek dosud vybrán a vrátí součet těchto hodnot.Případně s možnou hodnotou Null není žádná hodnota je ignorována.

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

minByNullable

Vybere s možnou NULL hodnotou pro každý prvek dosud vybrán a vrátí minimálně tyto hodnoty.Případně s možnou hodnotou Null není žádná hodnota je ignorována.

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

maxByNullable

Vybere s možnou NULL hodnotou pro každý prvek dosud vybrán a vrátí maximální hodnoty.Případně s možnou hodnotou Null není žádná hodnota je ignorována.

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

averageByNullable

Vybere s možnou NULL hodnotou pro každý prvek dosud vybrán a vrátí průměr těchto hodnot.Případně s možnou hodnotou Null není žádná hodnota je ignorována.

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

averageBy

Vybere hodnoty pro každý prvek dosud vybrán a vrátí průměr těchto hodnot.

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

distinct

Vybere rozdílné prvky z prvky vybrány tak daleko.

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

exists

Určuje, zda libovolný prvek vybrán zatím splňuje podmínku.

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

find

Vybere první prvek vybrán doposud splňující zadané podmínky.

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

all

Určuje, zda všechny prvky, které jsou vybrány zatím splňuje podmínku.

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

head

Vybere první prvek z ti tak daleko.

query {
    for student in db.Student do
    head
}

nth

Vybere element na zadaném indexu mimo vybraných tak daleko.

query {
    for numbers in data do
    nth 3
}

skip

Vynechá zadaný počet prvků, zatím vybrali a pak vybere zbývající prvky.

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

skipWhile

Vynechá prvky v sekvenci, tak dlouho, dokud zadaná podmínka pravdivá a pak vybere zbývající prvky.

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

sumBy

Vybere hodnoty pro každý prvek dosud vybrán a vrátí součet těchto hodnot.

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

take

Vybere zadaný počet souvislých prvků z vybraných tak daleko.

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

takeWhile

Vybere elementy z řady tak dlouho, dokud zadaná podmínka má hodnotu true a pak přeskočí zbývající prvky.

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

sortByNullable

Seřadí prvky zvolila doposud ve vzestupném pořadí dané s možnou hodnotou Null třídicím klíčem.

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

sortByNullableDescending

Seřadí prvky zvolila tak daleko v sestupném pořadí dané s možnou hodnotou Null třídicím klíčem.

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

thenByNullable

Provádí následné pořadí prvků zvolila doposud ve vzestupném pořadí dané s možnou hodnotou Null třídicím klíčem.Tento operátor lze použít pouze bezprostředně po sortBy, sortByDescending, thenBy, nebo thenByDescending, nebo jejich varianty s možnou hodnotou Null.

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

thenByNullableDescending

Provádí následné pořadí prvků zvolila tak daleko v sestupném pořadí dané s možnou hodnotou Null třídicím klíčem.Tento operátor lze použít pouze bezprostředně po sortBy, sortByDescending, thenBy, nebo thenByDescending, nebo jejich varianty s možnou hodnotou Null.

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

Porovnání Transact-SQL a výrazech dotazu F#

Následující tabulka uvádí některé běžné dotazy jazyka Transact-SQL a jejich ekvivalenty v F#.Kód v této tabulce také předpokládá stejné databázi jako v předchozí tabulce a stejné počáteční kód nastavit typ zprostředkovatele.

Tabulka 2.Transact-SQL a výrazech dotazu F#

Transact-SQL (nerozlišuje velikost písmen)

F# výrazu dotazu (jsou rozlišována malá a velká písmena)

Vyberte všechna pole z tabulky.

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

Počet záznamů v tabulce.

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
}

Seskupení

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())
}

Seskupení s podmínkou.

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())
}

Seskupení podmínce 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())
}

Seskupení, počítání a sčítání.

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)
}

Seskupování, počítání a řazení podle počtu.

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())
}

INSada zadaných hodnot

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 a 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   
    }

LIKEvzorkem odpovídají sadě.

-- '[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  
    }

LIKEvyloučení vzorkem sadu.

-- '[^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 jednom poli, ale vyberte jiné 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, s podřetězec hledání.

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
}

Jednoduchý JOIN s tabulkami.

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 JOINpomocí dvou tabulek.

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)
    }

JOINsCOUNT

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        
}

Přesný součet.

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
}

ORs řazení

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, ORa řazení.

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
    }

UNIONdvou dotazů.

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)

Průnik dvou dotazů.

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)

CASEpodmínka.

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))
    }

Více případů.

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))
    }

Více tabulek.

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

Vícenásobná spojení.

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)
    }

Více levé vnější spojení.

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)
    }

Chcete-li vytvořit ukázkovou databázi pro tyto příklady lze použít následující kód.

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);

Následující kód obsahuje ukázkový kód, který se zobrazí v tomto tématu.

#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 zde je úplný výstup, pokud je tento kód spuštěn v F# interaktivní.

Viz také

Referenční dokumentace

Třída LINQ.QueryBuilder (F#)

Další zdroje

F# Language Reference

Výpočet výrazy (F#)