SQL JOIN : Retrieving Data from many Tables

Joins are created when we display Data in queries that using more than one tables. we can connect a row in one table to another row in other table using the same value of both rows. Usualy this relation represented as Primary Key and Foreign Key relationship.

Joins allow an association between a field in one table or view with a field of the same data type in another table or view. Joins show how the data in one table relates to the data in another table. By using joins, data from two or more tables or queries based on logical relationships among the tables can be retrieved. Joins can be added, edited, or deleted to retrieve the data from the database.

For Example you have two tables called Products and Vendors that contain the following Data:

Data in table Vendors:

VendorID  VendorName                  URL
--------- --------------------------- --------------------------
V0001     Microsoft Corporation       http://www.microsoft.com
V0002     Oracle Corporation          http://www.oracle.com
V0003     MySQL AB                    http://www.mysql.com
V0004     PremiumSoft CyberTech Ltd.  http://www.navicat.com
V0005     Mentat Technologies         http://www.mentattech.com

Data in table Products:

ProductID  VendorID  Description
---------- --------- -----------------------------------------
SDB0001    V0001     MS SQL Server Database 2005
SDB0002    V0002     Oracle Database 10g Enterprise
SDB0003    V0003     MySQL 5.0 Enterprise Edition
SDBA0001   V0004     Navicat For MySQL Administration
SDBA0002   V0005     Dreamcoder 4.1 For MySQL Enterprise
SDBA0003   V0005     Dreamcoder 4.1 For MySQL Profesional
SDBA0004   V0005     Dreamcoder 4.1 For MS SQL Server Database

Bellow is SQL JOIN Statement example to display ProductID, VendorName, and Description by Joining two tables above


select products.ProductID,vendors.VendorName,products.Description

from vendors, products

where vendors.vendorid=products.vendorid

Query Result:

ProductID  VendorName                 Description
---------- -------------------------- -----------------------------------------
SDB0001    Microsoft Corporation      MS SQL Server Database 2005
SDB0002    Oracle Corporation         Oracle Database 10g Enterprise
SDB0003    MySQL AB                   MySQL 5.0 Enterprise Edition
SDBA0001   PremiumSoft CyberTech Ltd. Navicat For MySQL Administration
SDBA0002   Mentat Technologies        Dreamcoder 4.1 For MySQL Enterprise
SDBA0003   Mentat Technologies        Dreamcoder 4.1 For MySQL Profesional
SDBA0004   Mentat Technologies        Dreamcoder 4.1 For MS SQL Server Database

Or you can use this form of SQL JOIN, will produce the same result


select products.ProductID,vendors.VendorName,products.Description

from vendors inner join products

on vendors.vendorid=products.vendorid

Query Result:

ProductID  VendorName                  Description
---------- --------------------------- -----------------------------------------
SDB0001    Microsoft Corporation       MS SQL Server Database 2005
SDB0002    Oracle Corporation          Oracle Database 10g Enterprise
SDB0003    MySQL AB                    MySQL 5.0 Enterprise Edition
SDBA0001   PremiumSoft CyberTech Ltd.  Navicat For MySQL Administration
SDBA0002   Mentat Technologies         Dreamcoder 4.1 For MySQL Enterprise
SDBA0003   Mentat Technologies         Dreamcoder 4.1 For MySQL Profesional
SDBA0004   Mentat Technologies         Dreamcoder 4.1 For MS SQL Server Database