SQL joins

Introduction:

In this post I will explain what are the Joins in SQL Server and different types of Joins example (SQL LEFT outer Join, SQL RIGHT outer Join, SQL FULL outer Join, SQL Cross Join, SQL inner Join sample, Self Join example) and uses of Joins in SQL Server.

Description:

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	SureshDasari	Suresh	Dasari
2	PrasanthiDonthi	Prasanthi	Donthi
3	MaheshDasari	Mahesh	Dasari

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 query 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
SureshDasari	Dasari	543224
PrasanthiDonthi	Donthi	213424
MaheshDasari	Dasari	977776
MaheshDasari	Dasari	323233
SureshDasari	Dasari	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	SureshDasari	Suresh	Dasari	1	543224	1
2	PrasanthiDonthi	Prasanthi	Donthi	2	213424	2
3	MaheshDasari	Mahesh	Dasari	3	977776	3
3	MaheshDasari	Mahesh	Dasari	4	323233	3
1	SureshDasari	Suresh	Dasari	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	SureshDasari	Suresh	Dasari	1	543224	1
1	SureshDasari	Suresh	Dasari	2	213424	2
1	SureshDasari	Suresh	Dasari	3	977776	3
1	SureshDasari	Suresh	Dasari	4	323233	3
1	SureshDasari	Suresh	Dasari	5	998756	1
2	PrasanthiDonthi	Prasanthi	Donthi	1	543224	1
2	PrasanthiDonthi	Prasanthi	Donthi	2	213424	2
2	PrasanthiDonthi	Prasanthi	Donthi	3	977776	3
2	PrasanthiDonthi	Prasanthi	Donthi	4	323233	3
2	PrasanthiDonthi	Prasanthi	Donthi	5	998756	1
3	MaheshDasari	Mahesh	Dasari	1	543224	1
3	MaheshDasari	Mahesh	Dasari	2	213424	2
3	MaheshDasari	Mahesh	Dasari	3	977776	3
3	MaheshDasari	Mahesh	Dasari	4	323233	3
3	MaheshDasari	Mahesh	Dasari	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	SureshDasari	543224
1	SureshDasari	998756
2	PrasanthiDonthi	213424
3	MaheshDasari	977776
3	MaheshDasari	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 Column_List FROM table_name1 t1 RIGHT OUTER JOIN table_name2 t2 ON t1.column_name=t2.column_name

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	SureshDasari	543224
2	PrasanthiDonthi	213424
3	MaheshDasari	977776
3	MaheshDasari	323233
1	SureshDasari	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 RIGHT 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	SureshDasari	Suresh	Dasari	1	543224	1
1	SureshDasari	Suresh	Dasari	5	998756	1
2	PrasanthiDonthi	Prasanthi	Donthi	2	213424	2
3	MaheshDasari	Mahesh	Dasari	3	977776	3
3	MaheshDasari	Mahesh	Dasari	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	Suresh	2
2	Prasanthi	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	Suresh
Nagaraju	Suresh
Suresh	Prasanthi
Mahesh	Prasanthi
Mahendra	Mahesh
Sanjay	Mahesh
Prasanthi	Sai