Tabele PHP

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. 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

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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

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Referring to Two Tables

We can select data from two tables by referring to two tables, like this:

Example

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

SELECT Employees.Name, Orders.Product

FROM Employees, Orders

WHERE Employees.Employee_ID=Orders.Employee_ID