SQL RIGHT JOIN

As you have learned about the SQL LEFT JOIN where all the rows are returned from the left table ever when there is no match from the right table, the same case is with the SQL RIGHT JOIN, but here all the rows from the right side table is returned without matching the left side table. Let start with the syntax:

Syntax:-
SELECT column_name(s)
FROM table_name1
RIGHT JOIN table_name2
ON table_name1.column_name=table_name2.column_name;

Now look at the two tables, TABLE A and TABLE B:

TABLE A

P_Id	LastName	FirstName	Address	City
1	ABC	DEF	Street 10	City 1
2	GHI	JKL	Street 20	City 2
3	MNO	PQR	Street 30	City 3

TABLE B

O_Id	OrderNo	P_Id
1	778951	3
2	446781	3
3	224561	1
4	245621	1
5	347641	15

Now we apply the SQL RIGHT JOIN query:

SELECT TABLE A.LastName, TABLE A.FirstName, TABLE B.OrderNo
FROM TABLE A
RIGHT JOIN TABLE B
ON TABLE A.P_Id=TABLE B.P_Id
ORDER BY TABLE A.LastName;

The above query will return all the rows from TABLE A even there is no match with TABLE B. That does not mean that no check for common rows are made. The result of the above query will be:

OUTPUT

LastName	FirstName	OrderNo
ABC	DEF	224561
ABC	DEF	245621
MNO	PQR	778951
MNO	PQR	446781
		347641

NOTE: In some places SQL RIGHT JOIN is also refered as RIGHT OUTER JOIN.

SQL LEFT JOIN

This article expect that you have a prior knowledge of JOINING the table.

Before we start let start with the syntax of SQL LEFT JOIN, because this will help to understand the rest of the article quite easily

SQL LEFT JOIN Syntax
SELECT column_name(s)
FROM table_name1
LEFT JOIN table_name2
ON table_name1.column_name=table_name2.column_name;

The major part of the above syntax is " LEFT JOIN table_name2 ON table_name1.column_name=table_name2.column_name;"
Notice the keyword LEFT JOIN , what does this mean ? LEFT JOIN returns all the rows from left table even though the rows in the right table do not exist. To explain it clearly lets take an example:

TABLE A

P_Id	LastName	FirstName	Address	City
1	ABC	DEF	Street 10	City 1
2	GHI	JKL	Street 20	City 2
3	MNO	PQR	Street 30	City 3

TABLE B

O_Id	OrderNo	P_Id
1	778951	3
2	446781	3
3	224561	1
4	245621	1
5	347641	15

Now if we join two tables on the above provided Syntax, i.e.

SELECT TABLE A.LastName, TABLE A.FirstName, TABLE B.OrderNo
FROM TABLE A
LEFT JOIN Orders
ON TABLE A.P_Id=TABLE B.P_Id
ORDER BY TABLE A.LastName ;

Yes, you guessed it correctly that all the rows from the TABLE A will be returned without taking into consideration that those rows exits in the TABLE B

The OUTPUT will be

LastName	FirstName	OrderNo
ABC	DEF	224561
ABC	DEF	245621
MNO	PQR	778951
MNO	PQR	446781
		347641

NOTE: In some places SQL LEFT JOIN is also refered as LEFT OUTER JOIN.

INNER JOIN

Today let's discuss on the advance and the most important part that is how to join the two tables. Their are many ways to join a table but it is up to you what type of joining do you need The two type of joining is the INNER JOIN and OUTER JOIN.

Lets start with INNER JOIN.
By the term inner join we mean joining two or more table under some condition i.e through WHERE clause. It is just like a JOIN statement. Lets see the example

Syntax
SELECT column_name(s)
FROM table_name1
INNER JOIN table_name2
ON table_name1.column_name=table_name2.column_name

TABLE A

P_Id	LastName	FirstName	Address	City
1	ABC	DEF	Street 10	PPP
2	GHI	IJK	Street 20	QQQ
3	LMN	OPQ	Street 30	DDD

TABLE B

O_Id	OrderNo	P_Id
1	111111	1
2	222222	2
3	444444	3
4	555555	4
5	666666	5

Now we ask for INNER JOIN
SELECT Persons.LastName, Persons.FirstName, Orders.OrderNo
FROM TABLE A
INNER JOIN TABLE B
ON TABLE A.P_Id=TABLE B.P_Id
ORDER BY TABLE A.LastName

The out put that it will return will be

LastName	FirstName	OrderNo
ABC	DEF	111111
GHI	IJK	222222
LMN	OPQ	333333

The INNER JOIN keyword will return the number of rows that are common in both the table , like here the column that are returned are of two common people Hansen, Pettersen.

SQL Join Tables

In SQL join keyword is used quite offen. It is used to join two or more table together. There may be situation when you would like to join two or more table to get the common data or to verify some query.

Lets take this example.


This is a basic example and the complexities depend on the level of query that you want.

TABLE A

P_Id	LastName	FirstName	Address	City
1	ABC	DEF	Street 10	PPP
2	GHI	IJK	Street 20	QQQ
3	LMN	OPQ	Street 30	DDD

TABLE B

O_Id	OrderNo	P_Id
1	111111	1
2	222222	2
3	444444	3
4	555555	4
5	666666	5

To join a table, the most important thing to remember is to join the table on primary key. I am saying this because we want to join table on unique ID and primary key sufies this situation. As in the TABLE A table P_ID is unique ID and no two employees can have the same, so we join the table on this bases. If this condition is not taken into account the table will still join and provide the out put but that would be a catisian product i.e all the tables in TABLE A and all the tables in TABLE B. Another thing to remember is that both the tables should have common columb, on which you could join the table. As you can see TABLE A and TABLE B have P_ID is common, so the common table will be returned.

Syntax
SELECT column_name(s)
FROM table_name1
INNER JOIN table_name2
ON table_name1.column_name=table_name2.column_name;

For this example
SELECT P_ID
FROM TABLE A
INNER JOIN TABLE B
ON TABLE A.P_ID=TABLE B.P_ID;