SQL Server: EXISTS Và NOT EXISTS - V1Study

Các ví dụ

Các ví dụ dưới đây sẽ sử dụng cơ sở dữ liệu mẫu AdventureWorks.

A. Sử dụng NULL trong subquery nhưng vẫn trả về một tập kết quả

Ví dụ sau đây trả về một tập kết quả trong đó subquery chứa NULL nhưng EXISTS vẫn trả về TRUE.

USE AdventureWorks

SELECT DepartmentID, Name FROM HumanResources.Department WHERE EXISTS (SELECT NULL) ORDER BY Name ASC;

B. So sánh các truy vấn bằng cách sử dụng EXISTS và IN

Ví dụ sau đây so sánh hai truy vấn về mặt ngữ nghĩa là tương đương. Truy vấn đầu tiên sử dụng EXISTS và truy vấn thứ hai sử dụng IN.

USE AdventureWorks

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

Truy vấn sau đây sử dụng IN.

USE AdventureWorks

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

Dưới đây là tập kết quả mà cả hai câu truy vấn trên đều đưa ra như nhau.

FirstName LastName

-------------------------------------------------- ----------

Barry Johnson

David Johnson

Willis Johnson

(3 row(s) affected)

C. So sánh các truy vấn bằng cách sử dụng EXISTS và = ANY

Ví dụ sau đây thể hiện hai truy vấn đều nhằm mục đích tìm cửa hàng có tên là tên giống tên của nhà cung cấp. Truy vấn đầu tiên sử dụng EXISTS và truy vấn thứ hai sử dụng = ANY.

USE AdventureWorks

SELECT DISTINCT s.Name FROM Sales.Store AS s WHERE EXISTS (SELECT * FROM Purchasing.Vendor AS v WHERE s.Name = v.Name) ; GO

Truy vấn sau đây sử dụng = ANY.

USE AdventureWorks

SELECT DISTINCT s.Name FROM Sales.Store AS s WHERE s.Name = ANY (SELECT v.Name FROM Purchasing.Vendor AS v ) ; GO

D. So sánh các truy vấn bằng cách sử dụng EXISTS và IN

Ví dụ sau đây thể hiện các truy vấn tìm nhân viên của các phòng ban bắt đầu bằng 'P'.

USE AdventureWorks

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

Truy vấn sau đây sử dụng IN.

USE AdventureWorks

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

NOT EXISTS

NOT EXISTS ngược với EXISTS. Mệnh đề WHERE trong NOT EXISTS được thỏa mãn nếu subquery không trả về bất kỳ bản ghi nào. Ví dụ sau đây tìm những nhân viên không phải là người của phòng ban có tên bắt đầu bằng 'P'.

USE AdventureWorks

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

Dưới đây là tập hợp kết quả:

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)