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JOIN Operations in SQL
JOIN (typically INNER JOIN)
Definition: A JOIN operation in SQL, by default or when specified as an INNER JOIN, combines rows from two or more tables based on a related column between them, returning rows where there is at least one match in both tables.
Example:
-- INNER JOIN between Customers and Orders tables on the CustomerID
SELECT Customers.CustomerName, Orders.OrderID
FROM Customers
INNER JOIN Orders ON Customers.CustomerID = Orders.CustomerID;
Explanation: This query retrieves the customer name and order ID for all orders. If a customer has no orders, they do not appear in the result set. Conversely, if there are no matching customers for an order, that order is excluded.
LEFT JOIN
Definition: A LEFT JOIN (or LEFT OUTER JOIN) returns all rows from the left table, and the matched rows from the right table. If there is no match, the result is NULL on the side of the right table.
Example:
-- LEFT JOIN between Customers and Orders tables on the CustomerID
SELECT Customers.CustomerName, Orders.OrderID
FROM Customers
LEFT JOIN Orders ON Customers.CustomerID = Orders.CustomerID;
Explanation: This query includes all customers whether they have placed orders or not. For customers without orders, the OrderID will appear as NULL.
RIGHT JOIN
Definition: A RIGHT JOIN (or RIGHT OUTER JOIN) returns all rows from the right table, and the matched rows from the left table. If there is no match, the result is NULL on the side of the left table.
Example:
-- RIGHT JOIN between Customers and Orders tables on the CustomerID
SELECT Customers.CustomerName, Orders.OrderID
FROM Customers
RIGHT JOIN Orders ON Customers.CustomerID = Orders.CustomerID;
Explanation: This query includes all orders, whether there is a corresponding customer or not. For orders that do not have a corresponding customer, CustomerName will appear as NULL.
FULL OUTER JOIN
Definition: A FULL OUTER JOIN returns all rows when there is a match in either the left table or the right table. If there is no match, the result is NULL on the side of the table that does not have a match.
Example:
-- FULL OUTER JOIN between Customers and Orders tables on the CustomerID
SELECT Customers.CustomerName, Orders.OrderID
FROM Customers
FULL OUTER JOIN Orders ON Customers.CustomerID = Orders.CustomerID;
Explanation: This query includes all customers and all orders. It displays all matches where customers have orders and fills with NULLs where there is no match (i.e., customers without orders and orders without customers).
Each type of JOIN operation serves a specific purpose, depending on what you need from your data. INNER JOINs are used when you only want to retrieve records that have matching values in both tables. LEFT JOINs and RIGHT JOINs are helpful when you want to include all records from one side regardless of matches. FULL OUTER JOINs are used when you need a complete view of both sides, including all matches and all mismatches. These operations are fundamental in SQL for combining data from multiple tables in a relational database.