SQL Server JOINS Introduction

SQL Joins Introduction

The JOIN Clause is used in an SQL statement to query data from two or more tables, based on a relationship between certain columns in these tables. Tables in a database are often related to each other with keys.
A Primary Key is a column (or a combination of columns) with a unique value for each row. Each primary key value must be unique within the table. The purpose is to bind data together, across tables, without repeating all of the data in every table.

The "Customers" table:

Cust_Id	Cust_Name	Cust_Location
Cust_1234	ABC Soft Pvt Ltd.	East
Cust_3456	CDE Info Pvt Ltd	West
Cust_4466	FFDD Infor Pvt Ltd.	Mid-West
Cust_1122	EFC Tech Pvt Ltd.	South
Cust_6688	HHFF Tech Pvt Ltd.	Mid-South
Cust_5566	GHI Services Pvt Ltd.	North

Note that the "Cust_Id" column is the Primary Key in the "Customers" table. This means that two rows cannot have the same Cust_Id. The Cust_Id distinguishes two Customers even if they have the same name.

The "Orders" table:

Order_Id	Sales_Period	Cust_Id	Sales
ABC_1234	Q1-2014	Cust_1234	40474
BCD_2345	Q2-2014	Cust_2345	44321
CDE_3456	Q3-2014	Cust_3456	19906
DEF_4567	Q4-2014	Cust_4567	34943
EFG_1122	Q1-2015	Cust_1122	22114
FGH_3344	Q2-2015	Cust_3344	41364
GHI_5566	Q3-2015	Cust_5566	16157
IJK_7788	Q4-2015	Cust_7788	36054

Note that the "Order_Id" column is the Primary Key in the "Orders" table and that the "Cust_Id " column refers to the Customers in the "Customers" table without using their names.
Notice that the relationship between the two tables above is the "Cust_Id" column.


Different SQL JOINs:
Before we continue with examples, we will list the types of JOIN you can use, and the differences between them.

JOIN or INNER JOIN: 
Return rows when there is at least one match in both tables.

LEFT OUTER JOIN: 
Return all rows from the left table, even if there are no matches in the right table.

RIGHT OUTER JOIN: 
Return all rows from the right table, even if there are no matches in the left table.

FULL  OUTER JOIN: 
Return rows when there is a match in one of the tables.

Thanks, Tamatam