SQL JOINS

An SQL JOIN clause is used to fetch or extract records by multiple columns from two or more tables in a database, based on a common field between them.

Basically database tables are connected to each other with keys or values. We use these keys relationship to combine columns in our sql query.

The most common type of join is: SQL INNER JOIN (simple join). An SQL INNER JOIN return all rows from multiple tables where the join condition is met.

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Sample Database Table - Book1

BookId	BookName	Description
101	Sql Complete Reference	It descripes how to write and execute SQL statement into database.
102	Sql Commands	It exaplins a list of SQL command.
103	Pl Sql Quick Programming	How to write and execute SQL programming using pl sql.
104	Sql Query Injection	How to hack SQL queries in database.

Sample Database Table - Book2

BookId	AuthorName	DomainName	BookPrice
101	Suresh Babu	Sql Database	250.5
102	Siva Kumar	Sql Database	120
103	Azagu Varshith	Sql Programming	150
104	Bala Murugan	Sql Hacking	199.99

Note:- There must be a common columns while selecting columns from multiple tables. In the "Book1" and "Book2" both tables contains a common column called "BookID".

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SQL Joins - Example

Notice that the "BookID" column in the "Book1" table refers to the "BookID" in the "Book2" table. The relationship between the two tables above is the "BookID" column.

Then, if we run the following SQL statement (that contains an INNER JOIN):


SELECT Book1.BookId, Book1.BookName, Book2.AuthorName, Book2.BookPrice
FROM Book1 
INNER JOIN Book2 
ON Book1.BookId = Book2.BookId;

The result of above query is:

BookId	BookName	AuthorName	BookPrice
101	Sql Complete Reference	Suresh Babu	250.5
102	Sql Commands	Siva Kumar	120
103	Pl Sql Quick Programming	Azagu Varshith	150
104	Sql Query Injection	Bala Murugan	199.99

Different Types Of SQL Joins

There are different kinds of joins, which have different rules for the query results they create.

Below is a list of SQL Join types:

INNER JOIN: Returns all records when there is at least one match in BOTH tables
LEFT JOIN: Return all records from the left table, and the matched records from the right table
RIGHT JOIN: Return all records from the right table, and the matched records from the left table
FULL JOIN: Return all records when there is a match in ONE of the tables

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