Microsoft SQL Server
Vanliga tabelluttryck

Syntax

• MED cte_name [( column_name_1 , column_name_2 , ...)] AS ( cte_expression )

Anmärkningar

Medarbetarhierarki

Tabellinställning

CREATE TABLE dbo.Employees
(
    EmployeeID INT NOT NULL PRIMARY KEY,
    FirstName NVARCHAR(50) NOT NULL,
    LastName NVARCHAR(50) NOT NULL,
    ManagerID INT NULL
)

GO

INSERT INTO Employees VALUES (101, 'Ken', 'Sánchez', NULL)
INSERT INTO Employees VALUES (102, 'Keith', 'Hall', 101)
INSERT INTO Employees VALUES (103, 'Fred', 'Bloggs', 101)
INSERT INTO Employees VALUES (104, 'Joseph', 'Walker', 102)
INSERT INTO Employees VALUES (105, 'Žydrė', 'Klybė', 101)
INSERT INTO Employees VALUES (106, 'Sam', 'Jackson', 105)
INSERT INTO Employees VALUES (107, 'Peter', 'Miller', 103)
INSERT INTO Employees VALUES (108, 'Chloe', 'Samuels', 105)
INSERT INTO Employees VALUES (109, 'George', 'Weasley', 105)
INSERT INTO Employees VALUES (110, 'Michael', 'Kensington', 106)

Vanliga tabelluttryck

;WITH cteReports (EmpID, FirstName, LastName, SupervisorID, EmpLevel) AS
(
    SELECT EmployeeID, FirstName, LastName, ManagerID, 1
    FROM Employees
    WHERE ManagerID IS NULL

    UNION ALL

    SELECT e.EmployeeID, e.FirstName, e.LastName, e.ManagerID, r.EmpLevel + 1
    FROM Employees        AS e
    INNER JOIN cteReports AS r ON e.ManagerID = r.EmpID
)

SELECT
    FirstName + ' ' + LastName AS FullName,
    EmpLevel,
    (SELECT FirstName + ' ' + LastName FROM Employees WHERE EmployeeID = cteReports.SupervisorID) AS ManagerName
FROM cteReports
ORDER BY EmpLevel, SupervisorID

Produktion:

Fullständiga namn	EmpLevel	ManagerName
Ken Sánchez	1	null
Keith Hall	2	Ken Sánchez
Fred Bloggs	2	Ken Sánchez
Žydre Klybe	2	Ken Sánchez
Joseph Walker	3	Keith Hall
Peter Miller	3	Fred Bloggs
Sam Jackson	3	Žydre Klybe
Chloe Samuels	3	Žydre Klybe
George Weasley	3	Žydre Klybe
Michael Kensington	4	Sam Jackson

Hitta den nionde högsta lönen med CTE

Medarbetarnas tabell:

|  ID  | FirstName | LastName | Gender | Salary |
+------+-----------+----------+--------+--------+
| 1    | Jahangir  | Alam     | Male   | 70000  |
| 2    | Arifur    | Rahman   | Male   | 60000  |
| 3    | Oli       | Ahammed  | Male   | 45000  |
| 4    | Sima      | Sultana  | Female | 70000  |
| 5    | Sudeepta  | Roy      | Male   | 80000  |
+------+-----------+----------+--------+--------+

CTE (Common Table Expression):

 WITH RESULT AS
(
    SELECT SALARY,
           DENSE_RANK() OVER (ORDER BY SALARY DESC) AS DENSERANK
    FROM EMPLOYEES
)
SELECT TOP 1 SALARY
FROM RESULT
WHERE DENSERANK = 1

För att hitta den 2: a högsta lönen, ersätt helt enkelt N med 2. På samma sätt, för att hitta den 3: e högsta lönen, ersätt helt enkelt N med 3.

Ta bort duplicerade rader med CTE

Medarbetarnas tabell:

|  ID  | FirstName | LastName | Gender | Salary |
+------+-----------+----------+--------+--------+
|  1   | Mark      | Hastings | Male   | 60000  |
|  1   | Mark      | Hastings | Male   | 60000  |
|  2   | Mary      | Lambeth  | Female | 30000  |
|  2   | Mary      | Lambeth  | Female | 30000  |
|  3   | Ben       | Hoskins  | Male   | 70000  |
|  3   | Ben       | Hoskins  | Male   | 70000  |
|  3   | Ben       | Hoskins  | Male   | 70000  |
+------+-----------+----------+--------+--------+

CTE (Common Table Expression):

WITH EmployeesCTE AS
(
   SELECT *, ROW_NUMBER()OVER(PARTITION BY ID ORDER BY ID) AS RowNumber
   FROM Employees
)
DELETE FROM EmployeesCTE WHERE RowNumber > 1

Utföringsresultat:

|  ID  | FirstName | LastName | Gender | Salary |
+------+-----------+----------+--------+--------+
|  1   | Mark      | Hastings | Male   | 60000  |
|  2   | Mary      | Lambeth  | Female | 30000  |
|  3   | Ben       | Hoskins  | Male   | 70000  |
+------+-----------+----------+--------+--------+

Generera en tabell med datum med CTE

DECLARE @startdate CHAR(8), @numberDays TINYINT

SET @startdate = '20160101'
SET @numberDays = 10;

WITH CTE_DatesTable
AS
(
  SELECT CAST(@startdate as date) AS [date]
  UNION ALL
  SELECT DATEADD(dd, 1, [date])
  FROM CTE_DatesTable
  WHERE DATEADD(dd, 1, [date]) <= DateAdd(DAY, @numberDays-1, @startdate)
)

SELECT [date] FROM CTE_DatesTable

OPTION (MAXRECURSION 0)

Det här exemplet returnerar en tabell med en kolumn med datum, börjar med det datum som anges i variabeln @startdate och returnerar nästa datum för @numberDays.

Rekursiv CTE

Detta exempel visar hur man kommer varje år från i år till 2011 (2012 - 1).

WITH yearsAgo
(
    myYear
)
AS
(
     -- Base Case: This is where the recursion starts
     SELECT DATEPART(year, GETDATE()) AS myYear

     UNION ALL  -- This MUST be UNION ALL (cannot be UNION)

     -- Recursive Section: This is what we're doing with the recursive call
     SELECT yearsAgo.myYear - 1
     FROM yearsAgo
     WHERE yearsAgo.myYear >= 2012
)
     SELECT myYear FROM yearsAgo;  -- A single SELECT, INSERT, UPDATE, or DELETE

myYear
2016
2015
2014
2013
2012
2011

Du kan styra rekursionen (tänk stacköverskridning i kod) med MAXRECURSION som ett frågefunktion som kommer att begränsa antalet rekursiva samtal.

WITH yearsAgo
(
    myYear
)
AS
(
     -- Base Case
     SELECT DATEPART(year , GETDATE()) AS myYear
     UNION ALL
     -- Recursive Section
     SELECT yearsAgo.myYear - 1
     FROM yearsAgo
     WHERE yearsAgo.myYear >= 2002
)
     SELECT * FROM yearsAgo
     OPTION (MAXRECURSION 10);

Msg 530, nivå 16, del 1, rad 2Uttalandet upphörde. Den maximala rekursionen 10 har uttömts före uttalandet.

CTE med flera AS-uttalanden

;WITH cte_query_1
AS
(
    SELECT *
    FROM database.table1
), 
cte_query_2 
AS
(
    SELECT *
    FROM database.table2
)
SELECT *
FROM cte_query_1
WHERE cte_query_one.fk IN
(
    SELECT PK
    FROM cte_query_2
)

Med vanliga tabelluttryck är det möjligt att skapa flera frågor med kommaseparerade AS-uttalanden. En fråga kan sedan hänvisa till alla eller alla dessa frågor på många olika sätt, även gå med i dem.

Modified text is an extract of the original Stack Overflow Documentation

Licensierat under CC BY-SA 3.0

Inte anslutet till Stack Overflow