SQL SERVER – 2005 – Difference Between INTERSECT and INNER JOIN – INTERSECT vs. INNER JOIN

INTERSECT operator in SQL Server 2005 is used to retrieve the common records from both the left and the right query of the Intersect Operator. INTERSECT operator returns almost same results as INNER JOIN clause many times.

When using INTERSECT operator the number and the order of the columns must be the same in all queries as well data type must be compatible.

Let us see understand how INTERSECT and INNER JOIN are related.We will be using AdventureWorks database to demonstrate our example.

Example 1: Simple Example of INTERSECT

SELECT *
FROM HumanResources.EmployeeDepartmentHistory
WHERE EmployeeID IN (1,2,3)
INTERSECT
SELECT *
FROM HumanResources.EmployeeDepartmentHistory
WHERE EmployeeID IN (3,2,5)

ResultSet:

Explanation:
The ResultSet  shows  the  EmployeeID  which  are  common in both the Queries, i.e  2 and 3.

Example 2:  Using simple INTERSECTbetween two tables.

SELECT VendorID,ModifiedDate
FROM Purchasing.VendorContact
INTERSECT
SELECT VendorID,ModifiedDate
FROM Purchasing.VendorAddress

ResultSet:

Explanation:
The Resultset shows the records that are common in both the tables. It shows 104 common records between the tables.

Example 3:  Using INNER JOIN.

SELECT va.VendorID,va.ModifiedDate
FROM Purchasing.VendorContact vc
INNER JOIN Purchasing.VendorAddress va ON vc.VendorID = va.VendorID
AND vc.ModifiedDate = va.ModifiedDate

ResultSet:

Exlanation :
The resultset displays all the records which are common to both the tables. Additionally in example above INNER JOIN retrieves all the records from the left table and all the records from the right table. Carefully observing we can notice many of the records as duplicate records. When INNER JOIN is used it gives us duplicate records, but that is not in the case of INTERSECT operator.

Example 4:  Using INNER JOIN with Distinct.

SELECT DISTINCT va.VendorID,va.ModifiedDate
FROM Purchasing.VendorContact vc
INNER JOIN Purchasing.VendorAddress va ON vc.VendorID = va.VendorID
AND vc.ModifiedDate = va.ModifiedDate

ResultSet:

Explanation:
The resultset in this example does not contain any duplicate records as DISTINCT clause is used in SELECT statement. DISTINCT removes the duplicate rows and final result in this example is exactly same as example 2 described above. In this way, INNER JOIN can simulate with INTERSECT when used with DISTINCT.

Summary :

INNER JOIN can simulate with INTERSECT when used with DISTINCT.

Reference : Pinal Dave (https://blog.sqlauthority.com)