Monday, 11 February 2013

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.

Look at 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.


Next, we have 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.
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Thanks, TAMATAM ; Business Intelligence Professional
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