What are the all different types of Joins in SQL Server
2000, Anybody can explain each join with definition..Thanks
in advance....
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Answer Posted / unique fellow
Introduction
In this article, we’ll see the basic concepts of SQL
JOINs. In the later part of the article, we’ll focus on the
advanced subject of Self-JOIN and some interesting
observations on how inner JOIN can be simulated using left
JOIN. The author has tried his best to amalgamate various
topics in a single concept.
The JOIN keyword is used in a SQL statement to query data
from two or more tables based on a relationship between
certain columns in these tables.
Inner JOIN
A JOIN that displays only rows that have a match in both the
JOINed tables is known as inner JOIN. This is the default
type of JOIN in the Query and View Designer.
Outer JOIN
A JOIN that includes rows even if they do not have related
rows in the joined table is an Outer JOIN. You can create
three different outer JOINs to specify the unmatched rows to
be included:
Left Outer JOIN: In Left Outer JOIN, all rows in the
first-named table, i.e. “left” table, which appears leftmost
in the JOIN clause, are included. Unmatched rows in the
right table do not appear.
Right Outer JOIN: In Right Outer JOIN, all rows in the
second-named table, i.e. “right” table, which appears
rightmost in the JOIN clause, are included. Unmatched rows
in the left table are not included.
Full Outer JOIN: In Full Outer JOIN, all rows in all the
joined tables are included, whether they are matched or not.
Additional Notes related to JOIN
The following are three classic examples to demonstrate the
cases where Outer JOIN is useful. You must have noticed
several instances where developers write query as given below.
view sourceprint?
1.SELECT t1.*
2.FROM Table1 t1
3.WHERE t1.ID NOT IN (SELECT t2.ID FROM Table2 t2)
4.GO
The query demonstrated above can be easily replaced by Outer
JOIN. Indeed, replacing it by Outer JOIN is the best
practice. The query that generates the same result as above
is shown here using Outer JOIN and WHERE clause in JOIN.
view sourceprint?
1./* LEFT JOIN - WHERE NULL */
2.SELECT t1.*,t2.*
3.FROM Table1 t1
4.LEFT JOIN Table2 t2 ON t1.ID = t2.ID
5.WHERE t2.ID IS NULL
The above example can also be created using Right Outer JOIN.
NOT Inner JOIN
Remember, the term Not Inner JOIN does not exist in database
terminology. However, when full Outer JOIN is used along
with WHERE condition, as explained in the above two
examples, it will give you exclusive result to Inner JOIN.
This JOIN will show all the results that were absent in
Inner JOIN.
Cross JOIN
A cross JOIN devoid of a WHERE clause produces the Cartesian
product of the tables involved in the JOIN. The size of a
Cartesian product result set is the number of rows in the
first table multiplied by the number of rows in the second
table. One common example is when a company lists all its
products in a pricing table to compare each product with
others prices.
Self-JOIN
In this particular case, one table JOINs to itself with one
or two aliases to stave off confusion. A self-JOIN can be of
any type, as long as the joined tables are the same. A
self-JOIN is unique in the sense that it involves a
relationship with only one table. A common example is when a
company has a hierarchal reporting structure whereby a
member of staff reports to another member. Self-JOIN can
either be an Outer JOIN or an Inner JOIN.
Self-JOIN is accomplished by using table name aliases to
give each instance of the table a separate name. Joining a
table to itself can be useful when you want to compare
values in a column to other values of the same column.
Self-JOIN is a JOIN in which records from a table are
combined with other records from the same table when there
are matching values in the joined fields. A self-JOIN can
either be an inner JOIN or an outer JOIN. A table is joined
to itself based upon a field or combination of fields that
have duplicate data in different records. The data type of
the inter-related columns must be of the same type or cast
to the same type.
Now, think of a situation where all the data you require is
contained within a single table, but data needed to extract
is related to each other in the table itself. Examples of
this type of data relate to employee information, where the
table may have both an employee’s ID number for each record
and also a field that displays the ID number of an
employee’s supervisor or manager. To retrieve the data, it
is mandatory for the tables to relate/JOIN to itself.
Another example that can be tried on SQL SERVER 2005 sample
database AdventureWorks is to find products that are
supplied by more than one vendor. Please refer to the sample
database for table structure.
Note:
Before we continue further let me make it very clear that
INNER JOIN should be used where it cannot be used and
simulating INNER JOIN using any other JOINs will degrade the
performance. If there are scopes to convert any OUTER JOIN
to INNER JOIN, it should be done with priority.
Run the following two scripts and observe the result-set. It
will be identical.
view sourceprint?
01.USE AdventureWorks
02.GO
03./* Example of INNER JOIN */
04.SELECT p.ProductID, piy.ProductID
05.FROM Production.Product p
06.INNER JOIN Production.ProductInventory piy ON
piy.ProductID = p.ProductID
07.GO
08./* Example of LEFT JOIN simulating INNER JOIN */
09.SELECT p.ProductID, piy.ProductID
10.FROM Production.Product p
11.LEFT JOIN Production.ProductInventory piy ON 1 = 1
12.WHERE piy.ProductID = p.ProductID
13.GO
After gazing at the identical result the first question that
cropped up in my mind was - what is behind the scene plan?
Looking at the actual execution plan of the query it is
quite evident that even if LEFT JOIN is used in SQL Server
Query Optimizer, it converts to INNER JOIN since results are
the same and performance is better.
Looking at the above scenario it makes me ponder how smart
Query Optimizer Engine is and how it might be saving
innumerable performance-related issues for sub-optimal queries.
Now let us try to grasp the cause of LEFT JOIN acting as
INNER JOIN. When 1= 1 is used in ON clause it is always true
and converts LEFT JOIN to CROSS JOIN. However, when WHERE
condition’s effect is applied to the above CROSS JOIN it
produces a result similar to INNER JOIN in our case. SQL
Server Query Optimizer interprets this in advance and uses
INNER JOIN right away.
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