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
01 Hansen, Ola
02 Svendson, Tove
03 Svendson, Stephen
04 Pettersen, Kari
Prod_ID Product Employee_ID
234 Printer 01
657 Table 03
865 Chair 03
Referring to Two Tables
We can select data from two tables by referring to two tables, like this:
Who has ordered a product, and what did they order?
SELECT Employees.Name, Orders.Product
FROM Employees, Orders
Ai gasit ceva în neregulă cu acest document?