SQL JOIN

SQL Join is used to fetch data from two or more tables, which is joined to appear as single set of data. It is used for combining column from two or more tables by using values common to both tables.

JOIN Keyword is used in SQL queries for joining two or more tables. Minimum required condition for joining table, is (n-1) where n, is number of tables. A table can also join to itself, which is known as, Self Join.


Types of JOIN

Following are the types of JOIN that we can use in SQL:

  • Inner
  • Outer
  • Left
  • Right

Cross JOIN or Cartesian Product

This type of JOIN returns the cartesian product of rows from the tables in Join. It will return a table which consists of records which combines each row from the first table with each row of the second table.

Cross JOIN Syntax is,

SELECT column-name-list
FROM 
table-name1 CROSS JOIN table-name2;

Example of Cross JOIN

Following is the class table,

IDNAME
1abhi
2adam
4alex

and the class_info table,

IDAddress
1DELHI
2MUMBAI
3CHENNAI

Cross JOIN query will be,

SELECT * FROM 
class CROSS JOIN class_info;

The resultset table will look like,

IDNAMEIDAddress
1abhi1DELHI
2adam1DELHI
4alex1DELHI
1abhi2MUMBAI
2adam2MUMBAI
4alex2MUMBAI
1abhi3CHENNAI
2adam3CHENNAI
4alex3CHENNAI

As you can see, this join returns the cross product of all the records present in both the tables.


INNER Join or EQUI Join

This is a simple JOIN in which the result is based on matched data as per the equality condition specified in the SQL query.

Inner Join Syntax is,

SELECT column-name-list FROM 
table-name1 INNER JOIN table-name2 
WHERE table-name1.column-name = table-name2.column-name;

Example of INNER JOIN

Consider a class table,

IDNAME
1abhi
2adam
3alex
4anu

and the class_info table,

IDAddress
1DELHI
2MUMBAI
3CHENNAI

Inner JOIN query will be,

SELECT * from class INNER JOIN class_info where class.id = class_info.id;

The resultset table will look like,

IDNAMEIDAddress
1abhi1DELHI
2adam2MUMBAI
3alex3CHENNAI

Natural JOIN

Natural Join is a type of Inner join which is based on column having same name and same datatype present in both the tables to be joined.

The syntax for Natural Join is,

SELECT * FROM 
table-name1 NATURAL JOIN table-name2;

Example of Natural JOIN

Here is the class table,

IDNAME
1abhi
2adam
3alex
4anu

and the class_info table,

IDAddress
1DELHI
2MUMBAI
3CHENNAI

Natural join query will be,

SELECT * from class NATURAL JOIN class_info; 

The resultset table will look like,

IDNAMEAddress
1abhiDELHI
2adamMUMBAI
3alexCHENNAI

In the above example, both the tables being joined have ID column(same name and same datatype), hence the records for which value of ID matches in both the tables will be the result of Natural Join of these two tables.


OUTER JOIN

Outer Join is based on both matched and unmatched data. Outer Joins subdivide further into,

  1. Left Outer Join
  2. Right Outer Join
  3. Full Outer Join


LEFT Outer Join

The left outer join returns a resultset table with the matched data from the two tables and then the remaining rows of the left table and null from the right table's columns.

Syntax for Left Outer Join is,

SELECT column-name-list FROM 
table-name1 LEFT OUTER JOIN table-name2
ON table-name1.column-name = table-name2.column-name;

To specify a condition, we use the ON keyword with Outer Join.

Left outer Join Syntax for Oracle is,

SELECT column-name-list FROM 
table-name1, table-name2 on table-name1.column-name = table-name2.column-name(+);

Example of Left Outer Join

Here is the class table,

IDNAME
1abhi
2adam
3alex
4anu
5ashish

and the class_info table,

IDAddress
1DELHI
2MUMBAI
3CHENNAI
7NOIDA
8PANIPAT

Left Outer Join query will be,

SELECT * FROM class LEFT OUTER JOIN class_info ON (class.id = class_info.id);

The resultset table will look like,

IDNAMEIDAddress
1abhi1DELHI
2adam2MUMBAI
3alex3CHENNAI
4anunullnull
5ashishnullnull

RIGHT Outer Join

The right outer join returns a resultset table with the matched data from the two tables being joined, then the remaining rows of the right table and null for the remaining left table's columns.

Syntax for Right Outer Join is,

SELECT column-name-list FROM 
table-name1 RIGHT OUTER JOIN table-name2 
ON table-name1.column-name = table-name2.column-name;

Right outer Join Syntax for Oracle is,

SELECT column-name-list FROM 
table-name1, table-name2 
ON table-name1.column-name(+) = table-name2.column-name;

Example of Right Outer Join

Once again the class table,

IDNAME
1abhi
2adam
3alex
4anu
5ashish

and the class_info table,

IDAddress
1DELHI
2MUMBAI
3CHENNAI
7NOIDA
8PANIPAT

Right Outer Join query will be,

SELECT * FROM class RIGHT OUTER JOIN class_info ON (class.id = class_info.id);

The resultant table will look like,

IDNAMEIDAddress
1abhi1DELHI
2adam2MUMBAI
3alex3CHENNAI
nullnull7NOIDA
nullnull8PANIPAT

Full Outer Join

The full outer join returns a resultset table with the matched data of two table then remaining rows of both left table and then the right table.

Syntax of Full Outer Join is,

SELECT column-name-list FROM 
table-name1 FULL OUTER JOIN table-name2
ON table-name1.column-name = table-name2.column-name;

Example of Full outer join is,

The class table,

IDNAME
1abhi
2adam
3alex
4anu
5ashish

and the class_info table,

IDAddress
1DELHI
2MUMBAI
3CHENNAI
7NOIDA
8PANIPAT

Full Outer Join query will be like,

SELECT * FROM class FULL OUTER JOIN class_info ON (class.id = class_info.id);

The resultset table will look like,

IDNAMEIDAddress
1abhi1DELHI
2adam2MUMBAI
3alex3CHENNAI
4anunullnull
5ashishnullnull
nullnull7NOIDA
nullnull8PANIPAT