Oracle PL/SQL tutorial :SQL Left Join Example

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.