SQL Join

















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SQL Join




Joins and Keys


Sometimes we have to select data from two 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. The purpose is to bind data together, across tables, without
repeating all of the data in every
table.


In the "Employees" table below, the "ID" column is the
primary key, meaning that no two rows can have the same ID. The ID
distinguishes two
persons even if they have the same name.

When you look on the example tables below, notice that: 


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


Employees:


ID
Name


01
Hansen, Ola


02
Svendson, Tove


03
Svendson, Stephen


04
Pettersen, Kari



Orders:


ID
Product


01
Printer


03
Table


03
Chair





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


SELECT Employees.Name, Orders.Product
FROM Employees, Orders
WHERE Employees.ID = Orders.ID


Result


Name
Product


Hansen, Ola
Printer


Svendson, Stephen
Table


Svendson, Stephen
Chair



Example
Who ordered a printer?


SELECT Employees.Name
FROM Employees, Orders
WHERE Employees.ID = Orders.ID
AND Orders.Product = 'Printer'


Result


Name


Hansen, Ola







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