JOINS AND ITS TYPES WITH EXAMPLES

SQL JOINS :

In SQL joins are used to get data from two or more tables based on relationship between some of the columns in tables. In most of the cases we will use primary key of first table and foreign key of secondary table to get data from tables by using this relationship we can reduce the duplication of data in every table.

Before enter into Joins concept first design two tables in database and enter data like as shown below

Create one table with primary key and give name as “UserDetails”

UserID	UserName	FirstName	LastName
1	VamsiIndla	Vamsi	Indla
2	EkanathReddy	Ekanath	Reddy
3	MaheshVarma	Mahesh	Varma

Here UserID is the Primary key in UserDetails table

After that create another table with Foreign Key and give name as OrderDetails

OrderID	OrderNo	UserID
1	543224	1
2	213424	2
3	977776	3
4	323233	3
5	998756	1

Here OrderID is the Primary key and UserID is the foreign key in OrderDetails table.

SQL contains different types of Joins we will see each concept with example by 
using above tables.

Types of Joins

            1) Inner Joins

            2) Outer Joins

            3) Self Join

Inner Join

The join that displays only the rows that have a match in both the joined tables is 
known as inner join. This is default join in the query and view Designer.

Syntax for Inner Join

SELECT t1.column_name,t2.column_name

FROM table_name1 t1

INNER JOIN table_name2 t2

ON t1.column_name=t2.column_name

Now check the below quer for inner join

Example:

SELECT u.UserName,u.LastName,o.OrderNo

FROM UserDetails u

INNER JOIN OrderDetails o

ON u.UserID=o.UserID

Once we run that query our output will be like this

UserName	LastName	OrderNo
VamsiIndla	Indla	543224
EkanathReddy	Reddy	213424
MaheshVarma	Varma	977776
MaheshVarma	Varma	323233
VamsiIndla	Indla	998756

We can write our inner join query like this also it will give same result

SELECT u.UserName,u.LastName,o.OrderNo

FROM UserDetails u

JOIN OrderDetails o

ON u.UserID=o.UserID

Based on above result we can say that INNER JOIN keyword return rows when there is 
at least one match in both tables. If there are rows in "UserDetails" that do not have 
matches in "OrderDetails", those rows will NOT be listed.

In inner Join we are having different types of Joins those are

          1) Equi Join

2        2) Natural Join

           3) Cross Join

Equi Join

 The Equi join is used to display all the matched records from the joined tables and
 also display redundant values. In this join we need to use * sign to join the table.

Syntax for Equi Join

SELECT * FROM table_name1 t1

INNER JOIN table_name2 t2

ON t1.column_name=t2.column_name

Now check the below query for Equi join

Example

SELECT *

FROM UserDetails u

INNER JOIN OrderDetails o

ON u.UserID=o.UserID

Once we run above query our output will be like this

UserID	UserName	FirstName	LastName	OrderID	OrderNo	UserID
1	VamsiIndla	Vamsi	Indla	1	543224	1
2	EkanathReddy	Ekanath	Reddy	2	213424	2
3	MaheshVarma	Mahesh	Varma	3	977776	3
3	MaheshVarma	Mahesh	Varma	4	323233	3
1	VamsiIndla	Vamsi	Indla	5	998756	1

In equi join we need to use only equality comparisons in the join relation. If we use other operators such as (<,>) for our comparison condition then our Joins disqualifies for equi join.

Natural Joins

The Natural join is same as our Equi join but only the difference is it will restrict to display redundant values.

Syntax for Natural Join

SELECT * FROM table_name1 t1

NATURAL JOIN table_name2 t2

Example

SELECT *

FROM UserDetails

NATURAL JOIN OrderDetails

Note: These NATURAL Joins won’t work in our SQL Server (only supports in Oracle)
 it will throw syntax error. If you observe above code "NATURAL" is not highlighted, 
indicating
 that it is not recognized as a keyword.

Cross Join

A cross join that produces Cartesian product of the tables that involved in the join. The
 size of a Cartesian product is the number of the rows in first table multiplied by the number 
of rows in the second table.

Syntax for Cross Join

SELECT * FROM table_name1

CROSS JOIN table_name2

 Or we can write it in another way also

SELECT * FROM table_name1,table_name2

Now check the below query for Cross join

Example

SELECT * FROM UserDetails

CROSS JOIN OrderDetails

Or

SELECT * FROM UserDetails, OrderDetails

Once we run that query our output will be like this

UserID	UserName	FirstName	LastName	OrderID	OrderNo	UserID
1	VamsiIndla	Vamsi	Indla	1	543224	1
1	VamsiIndlai	Vamsi	Indla	2	213424	2
1	VamsiIndla	Vamsi	Indla	3	977776	3
1	VamsiIndla	Vamsi	Indla	4	323233	3
1	VamsiIndla	Vamsi	Indla	5	998756	1
2	EkanathReddy	Ekanath	Reddy	1	543224	1
2	EkanathReddy	Ekanath	Reddy	2	213424	2
2	EkanathReddy	Ekanath	Reddy	3	977776	3
2	EkanathReddy	Ekanath	Reddy	4	323233	3
2	EkanathReddy	Ekanath	Reddy	5	998756	1
3	MaheshVarma	Mahesh	Varma	1	543224	1
3	MaheshVarma	Mahesh	Varma	2	213424	2
3	MaheshVarma	Mahesh	Varma	3	977776	3
3	MaheshVarma	Mahesh	Varma	4	323233	3
3	MaheshVarma	Mahesh	Varma	5	998756	1

Outer Joins

A join that return all the rows that satisfy the condition and unmatched rows in the
 joined table is an Outer Join.

We are having three types of Outer Joins

            Left Outer Join

             Right Outer Join

             Full Outer Join

Left Outer Join

The left outer join displays all the rows from the first table and matched rows from the 
second table.

Syntax for Left Outer Join

SELECT Column_List FROM table_name1 t1

LEFT OUTER JOIN table_name2 t2

ON t1.column_name=t2.column_name

Now check the below query for Left Outer join

Example

SELECT u.UserID,u.UserName,o.OrderNo

FROM UserDetails u

LEFT OUTER JOIN OrderDetails o

ON u.UserID=o.UserID

Once we run that query our output will be like this

UserID	UserName	OrderNo
1	VamsiIndla	543224
1	VamsiIndla	998756
2	EkanathReddy	213424
3	MaheshVarma	977776
3	MaheshVarma	323233

Right Outer Join

The right outer join displays all the rows from the second table and matched rows from the first table.

Syntax for Right Outer Join

SELECT u.UserID,u.UserName,o.OrderNo

FROM UserDetails u

RIGHT OUTER JOIN OrderDetails o

ON u.UserID=o.UserID

Now check the below query for Right Outer join

Example

SELECT u.UserID,u.UserName,o.OrderNo

FROM UserDetails u

RIGHT OUTER JOIN OrderDetails o

ON u.UserID=o.UserID

Once we run that query our output will be like this

UserID	UserName	OrderNo
1	VamsiIndla	543224
2	EkanathReddy	213424
3	MaheshVarma	977776
3	MaheshVarma	323233
1	VamsiIndla	998756

Full Outer Join

Full Outer Join displays all the matching and non matching rows of both the tables.

Syntax for Full Outer Join

SELECT Column_List FROM table_name1 t1

FULL OUTER JOIN table_name2 t2

ON t1.column_name=t2.column_name

Now check the below query for Full Outer join

Example

SELECT u.UserID,u.UserName,o.OrderNo

FROM UserDetails u

FULL OUTER JOIN OrderDetails o

ON u.UserID=o.UserID

Once we run that query our output will be like this

UserID	UserName	FirstName	LastName	OrderID	OrderNo	UserID
1	VamsiIndla	Vamsi	Indla	1	543224	1
1	VamsiIndlai	Vamsi	Indla	5	998756	1
2	EkanathReddy	Ekanath	Reddy	2	213424	2
3	MaheshVarma	Mahesh	Varma	3	977776	3
3	MaheshVarma	Mahesh	Varma	4	323233	3

Self Join

Joining the table itself called self join. Self join is used to retrieve the records having some relation or similarity with other records in the same table. Here we need to use aliases for the same table to set a self join between single table and retrieve records satisfying the condition in where clause.

To implement self join first design table and give a name as “EmployeeDetails”

EmpID	EmpName	EmpMgrID
1	Vamsi	2
2	Ekanath	4
3	Mahesh	2
4	Sai	1
5	Nagaraju	1
6	Mahendra	3
7	Sanjay	3

Now I want to get manager names of particular employee for that we need to write 
query like this

select e2.EmpName,e1.EmpName as 'Manager'

from EmployeeDetails e1

INNER JOIN EmployeeDetails e2

on e1.EmpID=e2.EmpMgrID

Here if you observe above query EmployeeDetails table joined itself using table aliases
 e1 and e2.

After that run our query output will be like this

EmpName	Manger
Sai	Vamsi
Nagaraju	Vamsi
Suresh	Ekanath
Mahesh	Ekanath
Mahendra	Mahesh
Sanjay	Mahesh
Ekanath	Sai