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:
Example
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
