SQL Joins

A SQL JOIN is used to combine two or more tables, based on a related column between tables.

Let's take a look selection from the "Orders" table:

OrderId	CustomerId	OrderDate
1001	1	2020-05-22
1002	2	2020-05-22
1003	3	2020-05-22

Then, look at a selection from the "Customers" table:

CustomerId	CustomerName	Country
1	John	USA
2	Bill	USA
3	Jimmy	USA

Now, the relationship between Orders table and Customers table is the "CustomerId" column.

Then, we can create the following SQL statement that contains an INNER JOIN, It'll select records that have matching values in both tables:

SELECT o.OrderId, c.CustomerName, o.OrderDate
FROM Orders as o
INNER JOIN Customers as c ON o.CustomerId=c.CustomerId

and above SQL statement will produce following result like this:

OrderId	CustomerName	OrderDate
1001	John	2020-05-22
1002	Bill	2020-05-22
1003	Jimmy	2020-05-22

 

Types of SQL JOINs

1. INNER JOIN : Return records that have matching value in both tables.
2. LEFT OUTER JOIN : Returns all records from the left table, and the matched records from the right table
3. RIGHT OUTER JOIN : Returns all records from the right table, and the matched records from the left table
4. FULL OUTER JOIN : Returns all records when there is a match in either left or right table