SELF JOIN

Reference:http://blog.sqlauthority.com/2010/07/08/sql-server-the-self-join-inner-join-and-outer-join/




USE TempDb
GO
-- Create a Table
CREATE TABLE Employee(
EmployeeID INT PRIMARY KEY,
Name NVARCHAR(50),
ManagerID INT
)
GO
-- Insert Sample Data
INSERT INTO Employee
SELECT 1, 'Mike', 3
UNION ALL
SELECT 2, 'David', 3
UNION ALL
SELECT 3, 'Roger', NULL
UNION ALL
SELECT 4, 'Marry',2
UNION ALL
SELECT 5, 'Joseph',2
UNION ALL
SELECT 7, 'Ben',2
GO
-- Check the data
SELECT *
FROM Employee
GO

We will now use inner join to find the employees and their managers’ details.

-- Inner Join
SELECT e1.Name EmployeeName, e2.name AS ManagerName
FROM Employee e1
INNER JOIN Employee e2
ON e1.ManagerID = e2.EmployeeID
GO

From the result set, we can see that all the employees who have a manager are visible. However we are unable to find out the top manager of the company as he is not visible in our resultset. The reason for the same is that due to inner join, his name is filtered out. Inner join does not bring any result which does not have manager id. Let us convert Inner Join to Outer Join and then see the resultset.

-- Outer Join
SELECT e1.Name EmployeeName, ISNULL(e2.name, 'Top Manager') ASManagerName
FROM Employee e1
LEFT JOIN Employee e2
ON e1.ManagerID = e2.EmployeeID
GO

Once we convert Inner Join to Outer Join, we can see the Top Manager as well. Here we have seen how Self Join can behave as an inner join as well as an outer join.

As I said earlier, many of you know these details, but there are many who are still confused about this concept. I hope that this concept is clear from this post.

JOIN Tutorial

Thanks for sql blog authority
reference http://blog.sqlauthority.com/2009/04/13/sql-server-introduction-to-joins-basic-of-joins/

CREATE TABLE table1

(ID INT, Value VARCHAR(10))

INSERT INTO Table1 (ID, Value)

SELECT 1,'First'

UNION ALL

SELECT 2,'Second'

UNION ALL

SELECT 3,'Third'

UNION ALL

SELECT 4,'Fourth'

UNION ALL

SELECT 5,'Fifth'

GO

CREATE TABLE table2

(ID INT, Value VARCHAR(10))

INSERT INTO Table2 (ID, Value)

SELECT 1,'First'

UNION ALL

SELECT 2,'Second'

UNION ALL

SELECT 3,'Third'

UNION ALL

SELECT 6,'Sixth'

UNION ALL

SELECT 7,'Seventh'

UNION ALL

SELECT 8,'Eighth'

GO

SELECT *

FROM Table1

SELECT *

FROM Table2

GO

/* INNER JOIN */

SELECT t1.*,t2.*

FROM Table1 t1

INNER JOIN Table2 t2 ON t1.ID = t2.ID

GO

/* LEFT JOIN */

SELECT t1.*,t2.*

FROM Table1 t1

LEFT JOIN Table2 t2 ON t1.ID = t2.ID

GO

/* RIGHT JOIN */

SELECT t1.*,t2.*

FROM Table1 t1

RIGHT JOIN Table2 t2 ON t1.ID = t2.ID

GO

/* OUTER JOIN */

SELECT t1.*,t2.*

FROM Table1 t1

FULL OUTER JOIN Table2 t2 ON t1.ID = t2.ID

GO

/* LEFT JOIN - WHERE NULL */

SELECT t1.*,t2.*

FROM Table1 t1

LEFT JOIN Table2 t2 ON t1.ID = t2.ID

WHERE t2.ID IS NULL

GO

/* RIGHT JOIN - WHERE NULL */

SELECT t1.*,t2.*

FROM Table1 t1

RIGHT JOIN Table2 t2 ON t1.ID = t2.ID

WHERE t1.ID IS NULL

GO

/* OUTER JOIN - WHERE NULL */

SELECT t1.*,t2.*

FROM Table1 t1

FULL OUTER JOIN Table2 t2 ON t1.ID = t2.ID

WHERE t1.ID IS NULL OR t2.ID IS NULL

GO

/* CROSS JOIN */

SELECT t1.*,t2.*

FROM Table1 t1

CROSS JOIN Table2 t2

GO

DROP TABLE table1

DROP TABLE table2

GO