Introduction of SQL JOIN:

Many times you have to face the situation that you need data form two tables this time the “JOINs” are very useful. Joins are used to connect two tables and get the data from those connected tables.

In the database tabels are connected with each othe with “KEYS”. We can assisgn a PRIMERY key to the column and this column consist unique value for each row. The main purpose of creating Primary Key is to attach data together , beyond tables , without repeating 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 Tables:

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

Example:

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

Query:
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

Types of JOINs:

Here is the list of different SQL JOINs  that you can use :

  • INNER JOIN
  • LEFT JOIN
  • RIGHT JOIN
  • FULL JOIN

Note:

We will learn all this Joins in our next topic.

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