SQL学习：SQL JOIN

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Joins and Keys Sometimes we have to select data from two or more tables to make our result complete. We have to perform a join.
Tables in a database can be related to each other with keys. A primary key is a column with a unique value for each row. Each primary key value must be unique within the table. The purpose is to bind data together, across tables, without repeating all of the data in every table.
In the "Employees" table below, the "Employee_ID" column is the primary key, meaning that no two rows can have the same Employee_ID. The Employee_ID distinguishes two persons even if they have the same name.
When you look at the example tables below, notice that:  

• The "Employee_ID" column is the primary key of the "Employees" table
• The "Prod_ID" column is the primary key of the "Orders" table
• The "Employee_ID" column in the "Orders" table is used to refer to the persons in the "Employees" table without using their names

Employees:

Employee_ID	Name
01	Hansen, Ola
02	Svendson, Tove
03	Svendson, Stephen
04	Pettersen, Kari

Orders:

Prod_ID	Product	Employee_ID
234	Printer	01
657	Table	03
865	Chair	03

Referring to Two TablesWe can select data from two tables by referring to two tables, like this:
ExampleWho has ordered a product, and what did they order?

SELECT Employees.Name, Orders.ProductFROM Employees, OrdersWHERE Employees.Employee_ID=Orders.Employee_ID

Result

Name	Product
Hansen, Ola	Printer
Svendson, Stephen	Table
Svendson, Stephen	Chair

ExampleWho ordered a printer?

SELECT Employees.NameFROM Employees, OrdersWHERE Employees.Employee_ID=Orders.Employee_IDAND Orders.Product='Printer'

Result

Name

Hansen, Ola

Using JoinsOR we can select data from two tables with the JOIN keyword, like this:
Example INNER JOINSyntax

SELECT field1, field2, field3FROM first_tableINNER JOIN second_tableON first_table.keyfield = second_table.foreign_keyfield

Who has ordered a product, and what did they order?

SELECT Employees.Name, Orders.ProductFROM EmployeesINNER JOIN OrdersON Employees.Employee_ID=Orders.Employee_ID

The INNER JOIN returns all rows from both tables where there is a match. If there are rows in Employees that do not have matches in Orders, those rows will not be listed.
Result

Name	Product
Hansen, Ola	Printer
Svendson, Stephen	Table
Svendson, Stephen	Chair

Example LEFT JOINSyntax

SELECT field1, field2, field3FROM first_tableLEFT JOIN second_tableON first_table.keyfield = second_table.foreign_keyfield

List all employees, and their orders - if any.

SELECT Employees.Name, Orders.ProductFROM EmployeesLEFT JOIN OrdersON Employees.Employee_ID=Orders.Employee_ID

The LEFT JOIN returns all the rows from the first table (Employees), even if there are no matches in the second table (Orders). If there are rows in Employees that do not have matches in Orders, those rows also will be listed.
Result

Name	Product
Hansen, Ola	Printer
Svendson, Tove
Svendson, Stephen	Table
Svendson, Stephen	Chair
Pettersen, Kari

Example RIGHT JOINSyntax

SELECT field1, field2, field3FROM first_tableRIGHT JOIN second_tableON first_table.keyfield = second_table.foreign_keyfield

List all orders, and who has ordered - if any.

SELECT Employees.Name, Orders.ProductFROM EmployeesRIGHT JOIN OrdersON Employees.Employee_ID=Orders.Employee_ID

The RIGHT JOIN returns all the rows from the second table (Orders), even if there are no matches in the first table (Employees). If there had been any rows in Orders that did not have matches in Employees, those rows also would have been listed.
Result

Name	Product
Hansen, Ola	Printer
Svendson, Stephen	Table
Svendson, Stephen	Chair

ExampleWho ordered a printer?

SELECT Employees.NameFROM EmployeesINNER JOIN OrdersON Employees.Employee_ID=Orders.Employee_IDWHERE Orders.Product = 'Printer'

Result

Name

Hansen, Ola