EXISTS (Transact-SQL)
Specifica una sottoquery per testare l'esistenza di righe.
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Convenzioni della sintassi Transact-SQL
Sintassi
EXISTS subquery
Argomenti
- subquery
Istruzione SELECT con restrizioni. La parola chiave INTO non è consentita. Per ulteriori informazioni, vedere la descrizione delle sottoquery in SELECT (Transact-SQL).
Tipi di risultati
Boolean
Valori dei risultati
Restituisce TRUE se una sottoquery include una o più righe.
Esempi
A.Utilizzo di NULL in una sottoquery per restituire sempre un set di risultati
Nell'esempio seguente viene restituito un set di risultati grazie all'aggiunta di NULL nella sottoquery e viene comunque restituito TRUE tramite l'utilizzo della parola chiave EXISTS.
USE AdventureWorks2012 ;
GO
SELECT DepartmentID, Name
FROM HumanResources.Department
WHERE EXISTS (SELECT NULL)
ORDER BY Name ASC ;
B.Confronto di query con le parole chiave EXISTS e IN
Nell'esempio seguente vengono confrontate due query equivalenti da un punto di vista semantico. Nella prima query viene utilizzata la parola chiave EXISTS, mentre nella seconda query viene utilizzata la parola chiave IN.
USE AdventureWorks2012 ;
GO
SELECT a.FirstName, a.LastName
FROM Person.Person AS a
WHERE EXISTS
(SELECT *
FROM HumanResources.Employee AS b
WHERE a.BusinessEntityID = b.BusinessEntityID
AND a.LastName = 'Johnson');
GO
Nella query seguente viene utilizzata la parola chiave IN.
USE AdventureWorks2012 ;
GO
SELECT a.FirstName, a.LastName
FROM Person.Person AS a
WHERE a.LastName IN
(SELECT a.LastName
FROM HumanResources.Employee AS b
WHERE a.BusinessEntityID = b.BusinessEntityID
AND a.LastName = 'Johnson');
GO
Di seguito è riportato il set di risultati per entrambe le query.
FirstName LastName
-------------------------------------------------- ----------
Barry Johnson
David Johnson
Willis Johnson
(3 row(s) affected)
C.Confronto di query con le parole chiave EXISTS e = ANY
Nell'esempio seguente vengono illustrate due query per la ricerca di negozi il cui nome coincide con quello del fornitore. Nella prima query viene utilizzata la parola chiave EXISTS, mentre nella seconda query viene utilizzata la parola chiave = ANY.
USE AdventureWorks2012 ;
GO
SELECT DISTINCT s.Name
FROM Sales.Store AS s
WHERE EXISTS
(SELECT *
FROM Purchasing.Vendor AS v
WHERE s.Name = v.Name) ;
GO
Nella query seguente viene utilizzata la parola chiave = ANY.
USE AdventureWorks2012 ;
GO
SELECT DISTINCT s.Name
FROM Sales.Store AS s
WHERE s.Name = ANY
(SELECT v.Name
FROM Purchasing.Vendor AS v ) ;
GO
D.Confronto di query con le parole chiave EXISTS e IN
Nell'esempio seguente vengono illustrate query per la ricerca di dipendenti facenti parte di reparti il cui nome inizia con la lettera P.
USE AdventureWorks2012;
GO
SELECT p.FirstName, p.LastName, e.JobTitle
FROM Person.Person AS p
JOIN HumanResources.Employee AS e
ON e.BusinessEntityID = p.BusinessEntityID
WHERE EXISTS
(SELECT *
FROM HumanResources.Department AS d
JOIN HumanResources.EmployeeDepartmentHistory AS edh
ON d.DepartmentID = edh.DepartmentID
WHERE e.BusinessEntityID = edh.BusinessEntityID
AND d.Name LIKE 'P%');
GO
Nella query seguente viene utilizzata la parola chiave IN.
USE AdventureWorks2012;
GO
SELECT p.FirstName, p.LastName, e.JobTitle
FROM Person.Person AS p JOIN HumanResources.Employee AS e
ON e.BusinessEntityID = p.BusinessEntityID
JOIN HumanResources.EmployeeDepartmentHistory AS edh
ON e.BusinessEntityID = edh.BusinessEntityID
WHERE edh.DepartmentID IN
(SELECT DepartmentID
FROM HumanResources.Department
WHERE Name LIKE 'P%');
GO
E.Utilizzo della parola chiave NOT EXISTS
La parola chiave NOT EXISTS funziona in modo inverso rispetto a EXISTS. La clausola WHERE in NOT EXISTS viene soddisfatta se la sottoquery non restituisce alcuna riga. Nell'esempio seguente vengono cercati i dipendenti che non fanno parte di reparti il cui nome inizia con la lettera P.
SELECT p.FirstName, p.LastName, e.JobTitle
FROM Person.Person AS p
JOIN HumanResources.Employee AS e
ON e.BusinessEntityID = p.BusinessEntityID
WHERE NOT EXISTS
(SELECT *
FROM HumanResources.Department AS d
JOIN HumanResources.EmployeeDepartmentHistory AS edh
ON d.DepartmentID = edh.DepartmentID
WHERE e.BusinessEntityID = edh.BusinessEntityID
AND d.Name LIKE 'P%')
ORDER BY LastName, FirstName
GO
Set di risultati:
FirstName LastName Title
------------------------------ ------------------------------ ------------
Syed Abbas Pacific Sales Manager
Hazem Abolrous Quality Assurance Manager
Humberto Acevedo Application Specialist
Pilar Ackerman Shipping & Receiving Superviso
François Ajenstat Database Administrator
Amy Alberts European Sales Manager
Sean Alexander Quality Assurance Technician
Pamela Ansman-Wolfe Sales Representative
Zainal Arifin Document Control Manager
David Barber Assistant to CFO
Paula Barreto de Mattos Human Resources Manager
Shai Bassli Facilities Manager
Wanida Benshoof Marketing Assistant
Karen Berg Application Specialist
Karen Berge Document Control Assistant
Andreas Berglund Quality Assurance Technician
Matthias Berndt Shipping & Receiving Clerk
Jo Berry Janitor
Jimmy Bischoff Stocker
Michael Blythe Sales Representative
David Bradley Marketing Manager
Kevin Brown Marketing Assistant
David Campbell Sales Representative
Jason Carlson Information Services Manager
Fernando Caro Sales Representative
Sean Chai Document Control Assistant
Sootha Charncherngkha Quality Assurance Technician
Hao Chen HR Administrative Assistant
Kevin Chrisulis Network Administrator
Pat Coleman Janitor
Stephanie Conroy Network Manager
Debra Core Application Specialist
Ovidiu Crãcium Sr. Tool Designer
Grant Culbertson HR Administrative Assistant
Mary Dempsey Marketing Assistant
Thierry D'Hers Tool Designer
Terri Duffy VP Engineering
Susan Eaton Stocker
Terry Eminhizer Marketing Specialist
Gail Erickson Design Engineer
Janice Galvin Tool Designer
Mary Gibson Marketing Specialist
Jossef Goldberg Design Engineer
Sariya Harnpadoungsataya Marketing Specialist
Mark Harrington Quality Assurance Technician
Magnus Hedlund Facilities Assistant
Shu Ito Sales Representative
Stephen Jiang North American Sales Manager
Willis Johnson Recruiter
Brannon Jones Finance Manager
Tengiz Kharatishvili Control Specialist
Christian Kleinerman Maintenance Supervisor
Vamsi Kuppa Shipping & Receiving Clerk
David Liu Accounts Manager
Vidur Luthra Recruiter
Stuart Macrae Janitor
Diane Margheim Research & Development Enginee
Mindy Martin Benefits Specialist
Gigi Matthew Research & Development Enginee
Tete Mensa-Annan Sales Representative
Ramesh Meyyappan Application Specialist
Dylan Miller Research & Development Manager
Linda Mitchell Sales Representative
Barbara Moreland Accountant
Laura Norman Chief Financial Officer
Chris Norred Control Specialist
Jae Pak Sales Representative
Wanda Parks Janitor
Deborah Poe Accounts Receivable Specialist
Kim Ralls Stocker
Tsvi Reiter Sales Representative
Sharon Salavaria Design Engineer
Ken Sanchez Chief Executive Officer
José Saraiva Sales Representative
Mike Seamans Accountant
Ashvini Sharma Network Administrator
Janet Sheperdigian Accounts Payable Specialist
Candy Spoon Accounts Receivable Specialist
Michael Sullivan Sr. Design Engineer
Dragan Tomic Accounts Payable Specialist
Lynn Tsoflias Sales Representative
Rachel Valdez Sales Representative
Garrett Vargar Sales Representative
Ranjit Varkey Chudukatil Sales Representative
Bryan Walton Accounts Receivable Specialist
Jian Shuo Wang Engineering Manager
Brian Welcker VP Sales
Jill Williams Marketing Specialist
Dan Wilson Database Administrator
John Wood Marketing Specialist
Peng Wu Quality Assurance Supervisor
(91 row(s) affected)