Joins in MS SQL

Inner JOIN

A JOIN that displays only rows that have a match in both the JOINed tables is known as inner JOIN. This is the default type of JOIN in the Query.

Outer JOIN

A JOIN that includes rows even if they do not have related rows in the joined table is an Outer JOIN. You can create three different outer JOINs to specify the unmatched rows to be included:
Left Outer JOIN: In Left Outer JOIN, all rows in the first-named table, i.e. “left” table, which appears leftmost in the JOIN clause, are included. Unmatched rows in the right table do not appear.

Right Outer JOIN: In Right Outer JOIN, all rows in the second-named table, i.e. “right” table, which appears rightmost in the JOIN clause, are included. Unmatched rows in the left table are not included.

Full Outer JOIN: In Full Outer JOIN, all rows in all the joined tables are included, whether they are matched or not.

Cross JOIN

A cross JOIN devoid of a WHERE clause produces the Cartesian product of the tables involved in the JOIN. The size of a Cartesian product result set is the number of rows in the first table multiplied by the number of rows in the second table. One common example is when a company lists all its products in a pricing table to compare each product with others prices.

Self JOIN

In this example, we are actually self joining to the HumanResources.Employee table. We are doing this to obtain the information about the Employee and Manager relationship in the HumanResources.Employee table.

–creating table for self join

create table Employe

(

empID int Identity(1,1),

empName Varchar(15),

ManagerID int

)

 -self join for finding manager of a employee

SELECT e.empID ,e.empName as EmpName ,m.empName as ‘Manager name’

FROM Employe e JOIN Employe m

on  e.ManagerID=m.empID