025 SQL Joins

The current W3Schools page explains SQL Joins. Here’s a summary:

Purpose of SQL Joins:
Joins are used to combine rows from two or more tables based on a related column between them.
Example Tables:
The Orders table contains columns like OrderID, CustomerID, and OrderDate.
The Customers table includes CustomerID, CustomerName, ContactName, and Country.
The common column "CustomerID" relates the two tables.

Sample SQL JOIN Query:
Uses INNER JOIN:

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

Types of SQL JOINs:
- (INNER) JOIN: Returns records with matching values in both tables.
- LEFT (OUTER) JOIN: Returns all records from the left table and matched records from the right table.
- RIGHT (OUTER) JOIN: Returns all records from the right table and matched records from the left.
- FULL (OUTER) JOIN: Returns all records with a match in either left or right table.
Key takeaway:
The main purpose of an SQL JOIN is to combine rows from two or more tables based on a related column.[1]

SQL Joins

SQL JOIN

A JOIN clause is used to combine rows from two or more tables, based on a related column between them.

Let's look at a selection from the "Orders" table:

OrderID	CustomerID	OrderDate
10308	2	1996-09-18
10309	37	1996-09-19
10310	77	1996-09-20

Then, look at a selection from the "Customers" table:

CustomerID	CustomerName	ContactName	Country
1	Alfreds Futterkiste	Maria Anders	Germany
2	Ana Trujillo Emparedados y helados	Ana Trujillo	Mexico
3	Antonio Moreno Taquería	Antonio Moreno	Mexico

Notice that the "CustomerID" column in the "Orders" table refers to the "CustomerID" in the "Customers" table. The relationship between the two tables above is the "CustomerID" column.

Then, we can create the following SQL statement (that contains an INNER JOIN), that selects records that have matching values in both tables:

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

and it will produce something like this:

OrderID	CustomerName	OrderDate
10308	Ana Trujillo Emparedados y helados	9/18/1996
10365	Antonio Moreno Taquería	11/27/1996
10383	Around the Horn	12/16/1996
10355	Around the Horn	11/15/1996
10278	Berglunds snabbköp	8/12/1996

Different Types of SQL JOINs

Here are the different types of the JOINs in SQL:

(INNER) JOIN: Returns records that have matching values in both tables
LEFT (OUTER) JOIN: Returns all records from the left table, and the matched records from the right table
RIGHT (OUTER) JOIN: Returns all records from the right table, and the matched records from the left table
FULL (OUTER) JOIN: Returns all records when there is a match in either left or right table