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SQL Joins


SQL joins are used to combine rows from two or more tables.


SQL JOIN

An SQL JOIN clause is used to combine rows from two or more tables, based on a common field between them.

The most common type of join is: SQL INNER JOIN (simple join). An SQL INNER JOIN returns all rows from multiple tables where the join condition is met.

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

OrderID CustomerID OrderDate
101 2 1972-01-20
102 2 1996-09-19
103 3 1996-09-20

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

CustomerID CustomerName Phone Country
1 Ramiro K. Perez 354 469 2264 Iceland
2 Mable D. Eicher 354 458 4620 Iceland
3 Mary M. Kovar 14-533-3385 USA

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, if we run the following SQL statement (that contains an INNER JOIN):

Example

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

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 SQL JOINs

Before we continue with examples, we will list the types of the different SQL JOINs you can use:

  • INNER JOIN: Returns all rows when there is at least one match in BOTH tables
  • LEFT JOIN: Return all rows from the left table, and the matched rows from the right table
  • RIGHT JOIN: Return all rows from the right table, and the matched rows from the left table
  • FULL JOIN: Return all rows when there is a match in ONE of the tables