HI there, Please go tru this stuff ,I even added examples to get the concepts easily. SQL JOIN Joins and Keys Sometimes we have to select data from two or more tables to make our result complete. We have to perform a join. Tables in a database can be related to each other with keys. A primary key is a column with a unique value for each row. The purpose is to bind data together, across tables, without repeating all of the data in every table. In the "Employees" table below, the "Employee_ID" column is the primary key, meaning that no two rows can have the same Employee_ID. The Employee_ID distinguishes two persons even if they have the same name. When you look at the example tables below, notice that: * The "Employee_ID" column is the primary key of the "Employees" table * The "Prod_ID" column is the primary key of the "Orders" table * The "Employee_ID" column in the "Orders" table is used to refer to the persons in the "Employees" table without using their names Employees: Employee_ID Name 01 Hansen, Ola 02 Svendson, Tove 03 Svendson, Stephen 04 Pettersen, Kari Orders: Prod_ID Product Employee_ID 234 Printer 01 657 Table 03 865 Chair 03 Referring to Two Tables We can select data from two tables by referring to two tables, like this: Example Who has ordered a product, and what did they order? SELECT Employees.Name, Orders.Product FROM Employees, Orders WHERE Employees.Employee_ID=Orders.Employee_ID Result Name Product Hansen, Ola Printer Svendson, Stephen Table Svendson, Stephen Chair Example Who ordered a printer? SELECT Employees.Name FROM Employees, Orders WHERE Employees.Employee_ID=Orders.Employee_ID AND Orders.Product='Printer' Result Name Hansen, Ola Using Joins OR we can select data from two tables with the JOIN keyword, like this: Example INNER JOIN Syntax SELECT field1, field2, field3 FROM first_table INNER JOIN second_table ON first_table.keyfield = second_table.foreign_keyfield Who has ordered a product, and what did they order? SELECT Employees.Name, Orders.Product FROM Employees INNER JOIN Orders ON Employees.Employee_ID=Orders.Employee_ID The INNER JOIN returns all rows from both tables where there is a match. If there are rows in Employees that do not have matches in Orders, those rows will not be listed. Result Name Product Hansen, Ola Printer Svendson, Stephen Table Svendson, Stephen Chair Example LEFT JOIN Syntax SELECT field1, field2, field3 FROM first_table LEFT JOIN second_table ON first_table.keyfield = second_table.foreign_keyfield List all employees, and their orders - if any. SELECT Employees.Name, Orders.Product FROM Employees LEFT JOIN Orders ON Employees.Employee_ID=Orders.Employee_ID The LEFT JOIN returns all the rows from the first table (Employees), even if there are no matches in the second table (Orders). If there are rows in Employees that do not have matches in Orders, those rows also will be listed. Result Name Product Hansen, Ola Printer Svendson, Tove Svendson, Stephen Table Svendson, Stephen Chair Pettersen, Kari Example RIGHT JOIN Syntax SELECT field1, field2, field3 FROM first_table RIGHT JOIN second_table ON first_table.keyfield = second_table.foreign_keyfield List all orders, and who has ordered - if any. SELECT Employees.Name, Orders.Product FROM Employees RIGHT JOIN Orders ON Employees.Employee_ID=Orders.Employee_ID The RIGHT JOIN returns all the rows from the second table (Orders), even if there are no matches in the first table (Employees). If there had been any rows in Orders that did not have matches in Employees, those rows also would have been listed. Result Name Product Hansen, Ola Printer Svendson, Stephen Table Svendson, Stephen Chair Example Who ordered a printer? SELECT Employees.Name FROM Employees INNER JOIN Orders ON Employees.Employee_ID=Orders.Employee_ID WHERE Orders.Product = 'Printer' Result Name Hansen, Ola

There are three types of joins: Inner Join, Outer Join, Cross Join