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  • SQL Joins

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    The process of combining rows of two or more then two tables based on a common field between them are known as SQL joins.

    Types of SQL joins :

    INNER JOIN: It is returns rows only when at least one row matches from both tables.

    LEFT JOIN: It is return rows from the left table,which matched rows from the right table.

    RIGHT JOIN: It is return rows from the right table, which matched rows from the left table.

    FULL JOIN: It is return rows when there is a match in ONE of the tables.

    Here, I explain joins using below tables.

    "Orders" Table

    OrderID CustomerID OrderDate EmployeeID
    103 2 1996-09-18 1
    109 37 1996-09-19 2
    1010 77 1996-09-20 3



    "Customers" Table

    CustomerID CustomerName ContactName Country
    1 Chanda Chanda Rai Germany
    2 Babita Bob Mexico
    3 Manish Jon Mexico



    "Employees" Table

    EmployeeID LastName FirstName BirthDate
    1 Davolio Nancy 12/8/1968
    2 Fuller Andrew 2/19/1952
    3 Leverling Janet 8/30/1963



    INNER JOIN: The below SQL statement will return all customers with all orders:

    SELECT Orders.OrderID, Customers.CustomerName, Orders.OrderDate
    FROM Orders
    INNER JOIN Customers
    ON Orders.CustomerID=Customers.CustomerID; 
    

    LEFT JOIN: The below SQL statement will return all customers, and orders they might have:

    SELECT Customers.CustomerName, Orders.OrderID
    FROM Customers
    LEFT JOIN Orders
    ON Customers.CustomerID=Orders.CustomerID
    ORDER BY Customers.CustomerName; 
    

    RIGHT JOIN: The below SQL statement will return all customers, and orders they might have:

    SELECT Customers.CustomerName, Orders.OrderID
    FROM Customers
    LEFT JOIN Orders
    ON Customers.CustomerID=Orders.CustomerID
    ORDER BY Customers.CustomerName; 
    

    FULL JOIN: The below SQL statement will return all employees, and orders they have placed:

     SELECT Orders.OrderID, Employees.FirstName
    FROM Orders
    RIGHT JOIN Employees
    ON Orders.EmployeeID=Employees.EmployeeID
    ORDER BY Orders.OrderID;
    

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