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1. How to work with Subquery in Data Mining?
Data Miningis the only process of sorting the large data sets in a quick manner. Also, it
identifies each record with its relationship to solve through the data analysisskills.
Data miningincludes such wonderful methods for data analysis like Machine Learning,
Statistics, and Database Systems. The Main purpose of data mining is to extract the information
from large data set and transform it into a useful data structure.
In this article, we will learn about how subquery helps to find the better result with the help of data
mining but before going ahead we need to learn what is subquery and its working relationship with
data mining.
What is Subquery in Data Mining
A subquery is a query, which comes inside another query in SQL, this is also known as a Nested
query or Inner query. This returns data that is used in the main query on behalf of condition inside
parentheses. Therefore, A subquery is embedded within WHERE clause, FROM and SELECT
clause. You can see the exact syntax of a subquery as follows.

2. •A subquery always works within the WHERE Clause of other SQL SELECT command.
•It is always enclosed within parentheses.
•Also, you can use these operators, much like as >, <, or =.
•The nested query runs first before its main (Parent) query. So, the results of a nested
query pass to the outer query.
•You can not use ORDER BY command within nested query but you can use ORDER
BY command in the main query.
•An inner query always consists of one selected column in the SELECT clause until the
multiple columns compare to its selected columns.
•The BETWEEN clause cannot be used in the subquery, However, you can use
BETWEEn clause in the main query.
You can use all common commands like SELECT, INSERT, UPDATE and DELETE in
the subquery for getting the result better through the help of Data Mining Company process.
Subqueries with the SELECT Command
There is a common syntax for writing a subquery as given below,
SELECT column_name [, column_name ]

3. FROM table1 [, table2 ]
WHERE column_name OPERATOR
(SELECT column_name [, column_name ]
FROM table1 [, table2 ]
[WHERE]
)
So, let’s take a real example, we assume a table like as given below;
So, let’s start with a Select command for the following output,
SELECT * FROM CUSTOMERS
WHERE ID IN
(
SELECT ID FROM CUSTOMERS
WHERE SALARY > 4500
)
Finally, the output will look like this.

4. You have seen the simple example of insert the data in the table. Therefore, we take another
example for more clarification. We need to create two table tblProducts and tblProductSales and
insert sample data for the same.
Script to create table tblProducts
Create Table tblProducts
(
[Id] int identity primary key,
[Name] nvarchar(50),
[Description] nvarchar(250)
)
Create Table tblProductSales
(
Id int primary key identity,
ProductId int foreign key references tblProducts(Id),
UnitPrice int,
QuantitySold int
)
statement 1

5. Insert into tblProducts values ('TV', '52 inch black color LCD
TV')
statement 2
Insert into tblProducts values ('Laptop', 'Very thin black color
acer laptop')
statement 3
Insert into tblProducts values ('Desktop', 'HP high performance
desktop')
statement 4
Insert into tblProductSales values(3, 450, 5)
statement 5
Insert into tblProductSales values(2, 250, 7)
statement 6
Insert into tblProductSales values(3, 450, 4)
statement 7
Insert into tblProductSales values(3, 450, 9)
Finally, the output for the following table.

6. Now, I want to retrieve products that are not at all sold,
Select [Id], [Name], [Description]
from tblProducts
where Id not in (Select Distinct ProductId from tblProductSales)
Finally, the output as follows.
How Subquery works with Joins
By usinHow Subquery workss you the same results. So, Let’s create the query for the same as
follows,
Select tblProducts.[Id], [Name], [Description]
from tblProducts
left join tblProductSales
on tblProducts.Id = tblProductSales.ProductId
where tblProductSales.ProductId IS NULL
When you execute the code, you get the same output that we achieved above.

7. Hence, you have seen how to use a subquery with WHERE clause.
In addition, let’s have a look, a subquery in the SELECT clause. So, I want to retrieve the NAME
and TOTALQUANTITY sold by using a subquery.
Select [Name],
(Select SUM(QuantitySold) from tblProductSales where ProductId =
tblProducts.Id) as TotalQuantity
from tblProducts
order by Name
Hence, you get the same output.
If you use joins, then you can get the same out as above,
Select [Name], SUM(QuantitySold) as TotalQuantity
from tblProducts
left join tblProductSales
on tblProducts.Id = tblProductSales.ProductId
group by [Name]
order by Name
Finally, you get the Output:

8. From above examples, it seems clear now that, a subquery simply use a select statement, that gives
a single value in return and also inner query inside a SELECT, UPDATE, INSERT, or DELETE
command.
What to choose SubQueries or Joins in Data Mining
This is something very confusing because some of the developers get confused which one should be
used for sorting the data in data mining. But remember both the function gives the same result but
they have their own capabilities so let’s understand through a table.
we assume data in both the tables tblProducts and tblProductSales.
400,00 data in the tblProducts table
600,00 data in tblProductSales tables
When I execute the command, then, you get 306,199 rows in 6 seconds
Select Id, Name, Description
from tblProducts
where ID IN
(
Select ProductId from tblProductSales
)
Finally, you get the output as follows,

9. For this reason, please swipe the command and execute plan cache by using the following T-SQL
statement.
CHECKPOINT;
GO
DBCC DROPCLEANBUFFERS; -- Clears query cache
Go
DBCC FREEPROCCACHE; -- Clears execution plan cache
GO
Now, execute the command by using joins. Note that we get the exact same 306,199 rows in 6
seconds.
Select distinct tblProducts.Id, Name, Description
from tblProducts

10. inner join tblProductSales
on tblProducts.Id = tblProductSales.ProductId
Please Note: We are using automated SQL script to insert large amount of random data.
According to MSDN, a join gives better performance. Else, the subquery must be passed for each
result of the main query. As a result, a joins gives the better results instead of a subquery.
The above query command gives the products result that we have not sold for once. When I run the
query I get 93,801 rows in 3 seconds
Select Id, Name, [Description]
from tblProducts
where Not Exists(Select * from tblProductSales where ProductId =
tblProducts.Id)
Finally, you get the output as follows,

11. When I run the query by using joins, I get the same result 93,801 rows in 3 seconds.
Select tblProducts.Id, Name, [Description]
from tblProducts
left join tblProductSales
on tblProducts.Id = tblProductSales.ProductId
where tblProductSales.ProductId IS NULL

12. In conclusion, joins work faster than subqueries, but if we see in reality, it all depends on the plan of
execution that is made by SQL Server. Hence, if you want to get the output in a quick manner or
you do an extraction of data then go with joins for data mining.
Conclusion
In this article, you may find both results set with subquery and use of joins instead of using the
subquery while working in mining the data with any analytic tool like SQL, Hadoop, and Big Data.
A subquery verifies two identities with proper format for the IN clause. But, if we talk about a large
number of data in the database server, use joins queries rather than a subquery because joins takes
less time to get the output in data mining.
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