Learn From - Interactive SQL

SQL Joins are used to get data from two or more tables using the common values across the tables.
Tables are usually related to each other using Primary Key and Foreign Key.

Primary Key is one or more columns that uniquely represent a record.
Foreign Key is a column that is a Primary key in other table.

Joins are usually done using this Foreign Key and Primary Key to get related data.

The data in the tables are stored using Primary Key and Foreign Key relation to avoid redundancy.
This is to avoid same data getting stored repeatedly in the tables and making it difficult to update.

Below is shown the "Customers" table:


In the Customer table "CustomerID" is the Primary Key that acts as a unique value. You can see that the "CustomerID" column value does not repeat.

Below is shown the "Orders" table:


In the Orders table the "OrderID" is the Primary Key. Here you can see that the "CustomerID" column is the Foreign Key.

The Customer and the Orders table are related to each other using the Primary Key and Foreign Key relation.

Different Joins

a.	Inner Join
b.	Left Join
c.	Right Join
d.	Full Join