SQL JOIN connection details and simple use instances, sqljoin

SQL JOIN connection details and simple use instances, sqljoin

SQL JOIN connection

The SQL JOIN clause is used to combine rows from two or more tables based on the common fields between these tables.

The most common JOIN type: SQL INNER JOIN (simple JOIN ). SQL INNER JOIN returns all rows that meet the JOIN conditions from multiple tables.

Let's take a look at the data selected from the "Orders" table:

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

Then, check the data selected from the "MERs" table:

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

Note that the "mermerid" column in the "Orders" table points to the Customers in the "Customers" table. The above two tables are linked through the "mermerid" column.

Then, if we run the following SQL statement (including INNER JOIN ):

SELECT Orders.OrderID, Customers.CustomerName, Orders.OrderDateFROM OrdersINNER JOIN CustomersON Orders.CustomerID=Customers.CustomerID;

The running result is as follows:

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

Different SQL JOIN

Before continuing to explain the examples, we will first list the different SQL JOIN types you can use:

• INNER JOIN: If the table has at least one match, the row is returned.
• Left join:Returns all rows from the left table even if no match exists in the right table.
• Right join:Returns all rows from the right table even if no match exists in the left table.
• Full join:If one of the tables matches, the row is returned.

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