SQL Training

November 9, 2012 by Joey Blue Leave a Comment

How to Remove Duplicate Rows in SQL Server

Video: How to Remove Duplicate Rows in SQL Server – SQL Training Online
In this video, I show you a trick on removing duplicate rows in SQL Server 2012.

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How to Remove Duplicate Rows in SQL Server?

I first need a table that has duplicate records to remove, so here is a script to create some duplicates in an email address table.

create table email_list
(
name varchar(100)
,email varchar(300)
)
go
insert into email_list values ('Jan','[email protected]');
insert into email_list values ('Bob','[email protected]');
insert into email_list values ('Jill','[email protected]');
insert into email_list values ('Bob','[email protected]');
insert into email_list values ('Jill','[email protected]');
insert into email_list values ('Bob','[email protected]');
insert into email_list values ('Jill','[email protected]');

Now, the first method to remove the duplicates is in the following script that I adapted from Alexander’s Post on duplicates.

WITH list_rownumbers AS
(
SELECT name,email,
ROW_NUMBER() OVER (ORDER BY name,email) AS 'RowNumber'
FROM email_list
)
DELETE list_rownumbers WHERE RowNumber not in
(SELECT min(RowNumber) FROM list_rownumbers GROUP BY name,email)

If you just look at the SQL With statement (CTE), you will see that it simply numbers all of the name, email combinations using the Row_Number function. Then it uses the minimum rows number per name, email group as an exclusion to remove the duplicate rows.

The second script is and adaptation of Ritesh’s Post on duplicates.

WITH list_duplicates (name, email, duplicate_count) AS
(
SELECT name,email,
ROW_NUMBER() OVER(PARTITION BY name,email ORDER BY name,email) AS duplicate_count
FROM email_list
)
DELETE
FROM list_duplicates
WHERE duplicate_count > 1

If we focus on the CTE query, we see that is uses the Partition By statement to actually reset the row_number count on each name, email group.

It makes the delete statement a little easier to follow since all we have to do is delete where the row_number (duplicate_count) is larger than 1.

And that’s it. That is how you Delete Duplicates in SQL.

Let me know what you think by commenting or sharing on twitter, facebook, google+, etc.

SQL Training Online: https://www.sqltrainingonline.com

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November 5, 2012 by Joey Blue Leave a Comment

How to Filter for SQL Null or Empty String

Video: How to Filter for SQL Null or Empty String – SQL Training Online
In this video, I show you how to filter a SQL table that has both Null values and an Empty string. I use SQL Server 2012 to demonstrate the technique.

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How do you filter a SQL Null or Empty String?

A null value in a database really means the lack of a value. It is a special “value” that you can’t compare to using the normal operators. You have to use a clause in SQL IS Null.

On the other hand, an empty string is an actual value that can be compared to in a database. You simply use two ticks together.

''

Let’s take a look at the vendor table to demonstrate this.

select * from vendor

In this table, I specifically put in some email addresses that are both null and empty strings.

Let’s pull out the NULL values using the IS NULL operator.

select * from vendor
where vendor_email is null

Next, I want to pull out the empty string using the tick-tick, or empty string.

select * from vendor
where vendor_email = ''

If you want to combine them to search for the SQL null or empty string together and retrieve all of the empty strings and nulls all at once, you could do something like this.

select * from vendor
where vendor_email = ''
or vendor_email is null

Finally, I want to show you a little trick that I have used on occasion to change all of the null values to empty strings and then compare the result to an empty string.

select * from vendor
where isnull(vendor_email,'') = ''

And that is it.

Here is the script to create the vendor table I was using as an example.

create table dbo.vendor
(
vendor_id int identity(1,1) primary key,
vendor_name varchar(50),
vendor_email varchar(100)
)
insert into vendor (vendor_name,vendor_email) values ('Dell','[email protected]');
insert into vendor (vendor_name,vendor_email) values ('IBM','[email protected]');
insert into vendor (vendor_name,vendor_email) values ('Microsoft','[email protected]');
insert into vendor (vendor_name,vendor_email) values ('Oracle','');
insert into vendor (vendor_name,vendor_email) values ('Intel',NULL);
insert into vendor (vendor_name,vendor_email) values ('Apple','[email protected]');

There it is. That is how you filter for SQL Null or Empty String.

Let me know what you think by commenting or sharing on twitter, facebook, google+, etc.

If you enjoy the video, please give it a like, comment, or subscribe to my channel.

You can visit me at any of the following:

SQL Training Online: https://www.sqltrainingonline.com

Twitter: http://www.twitter.com/sql_by_joey

Google+: https://plus.google.com/#100925239624117719658/posts

LinkedIn: http://www.linkedin.com/in/joeyblue

Facebook: http://www.facebook.com/sqltrainingonline

November 1, 2012 by Joey Blue Leave a Comment

How to Test SQL Server Functions

Video: How to Test SQL Server Functions – SQL Training Online
In this video, I show you a trick on how to test SQL Functions in SQL Server 2012.

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How do you test a SQL Server Function?

Testing a new function is easier to do when you don’t actually try to query a table to do it.

I will show you how to trim off the front part of an email address by first testing the functions, then putting them into the actual SQL query.

Let’s start with the following SQL query that is written against the Adventure Works database.

SELECT
s.[BusinessEntityID]
,s.[Name]
,ct.[Name] AS [ContactType]
,p.[Title]
,p.[FirstName]
,p.[MiddleName]
,p.[LastName]
,p.[Suffix]
,pp.[PhoneNumber]
,pnt.[Name] AS [PhoneNumberType]
,ea.[EmailAddress]
,p.[EmailPromotion]
FROM [Sales].[Store] s
INNER JOIN [Person].[BusinessEntityContact] bec
ON bec.[BusinessEntityID] = s.[BusinessEntityID]
INNER JOIN [Person].[ContactType] ct
ON ct.[ContactTypeID] = bec.[ContactTypeID]
INNER JOIN [Person].[Person] p
ON p.[BusinessEntityID] = bec.[PersonID]
LEFT OUTER JOIN [Person].[EmailAddress] ea
ON ea.[BusinessEntityID] = p.[BusinessEntityID]
LEFT OUTER JOIN [Person].[PersonPhone] pp
ON pp.[BusinessEntityID] = p.[BusinessEntityID]
LEFT OUTER JOIN [Person].[PhoneNumberType] pnt
ON pnt.[PhoneNumberTypeID] = pp.[PhoneNumberTypeID];

This query joins multiple tables and can be intimidating to the beginner. The main thing to look at is that it returns an email address.

I want to trim off the front part of the email address.

So, I will copy an email address to work on as an example.

‘[email protected]’

Now, SQL Server allows you to run a Select query without a FROM clause.

We will use that capability to test our function. Here is an example.

select '[email protected]'

Now, I want to find the ‘@’ character in the string, so I will use the CharIndex function. Now it is time to test the SQL Server Function:

select charindex('@','[email protected]')

This returns the number 9, since the ‘@’ sign is the 9^th character.

Now, we need to use the substring to pull off all of the character until the ‘@’ sign. So, let’s test the substring function:

select charindex('@','[email protected]')
,substring('[email protected]',1,9-1)

This does what we want, so now we need to test both SQL Server Functions together.

select substring('[email protected]',1,charindex('@','[email protected]')-1)

This is exactly what we want. So, we just need to put the function into the main query.

SELECT
s.[BusinessEntityID]
,s.[Name]
,ct.[Name] AS [ContactType]
,p.[Title]
,p.[FirstName]
,p.[MiddleName]
,p.[LastName]
,p.[Suffix]
,pp.[PhoneNumber]
,pnt.[Name] AS [PhoneNumberType]
,ea.[EmailAddress]
,substring(ea.[EmailAddress] ,1,charindex('@',ea.[EmailAddress] )-1)
,p.[EmailPromotion]
FROM [Sales].[Store] s
INNER JOIN [Person].[BusinessEntityContact] bec
ON bec.[BusinessEntityID] = s.[BusinessEntityID]
INNER JOIN [Person].[ContactType] ct
ON ct.[ContactTypeID] = bec.[ContactTypeID]
INNER JOIN [Person].[Person] p
ON p.[BusinessEntityID] = bec.[PersonID]
LEFT OUTER JOIN [Person].[EmailAddress] ea
ON ea.[BusinessEntityID] = p.[BusinessEntityID]
LEFT OUTER JOIN [Person].[PersonPhone] pp
ON pp.[BusinessEntityID] = p.[BusinessEntityID]
LEFT OUTER JOIN [Person].[PhoneNumberType] pnt
ON pnt.[PhoneNumberTypeID] = pp.[PhoneNumberTypeID];

There it is. That is how you Test SQL Server Functions.

Let me know what you think by commenting or sharing on twitter, facebook, google+, etc.

If you enjoy the video, please give it a like, comment, or subscribe to my channel.

You can visit me at any of the following:

SQL Training Online: https://www.sqltrainingonline.com

Twitter: http://www.twitter.com/sql_by_joey

Google+: https://plus.google.com/#100925239624117719658/posts

LinkedIn: http://www.linkedin.com/in/joeyblue

Facebook: http://www.facebook.com/sqltrainingonline

October 29, 2012 by Joey Blue Leave a Comment

How to use the SQL In Statement with Subquery

Video: How to use the SQL In Statement with Subquery – SQL Training Online
In this video, I show you how to use the SQL IN Statement with Subquery in SQL Server 2012 using SQL Server Management Studio.

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How do you use the SQL IN Statement with a Subquery?

I will show you in 3 steps that build on each other:

Use the SQL OR operator.
Use the SQL IN operator.
Substitute a SQL Subquery into the IN Clause.

I will start with a simple query of the SQL Training Online Simple DB employee table where I want to retrieve 3 employee numbers: 7369,7499,7521

select * from employee
where employee_number=7369
or employee_number=7499
or employee_number=7521

Here, you can see that I used the SQL OR statement to get the 3 employees.

But, I can make this a little cleaner by using the SQL IN statement:

select * from employee
where employee_number in (7369,7499,7521)

This query returns the same data as before, but it has less typing.

So, now I want to define a subquery. It is simply:

“A subquery is a query inside of another query.”

Let’s start the example. First, I will look at the customer table and specifically, the salesman employee number:

select saleman_employee_number from customer

This query returns a list of employee numbers attached to a customer. But, I want to filter out the nulls and get rid of the repeated employee numbers. So, here is the query for that.

select distinct saleman_employee_number from customer
where saleman_employee_number is not null

Now, this is where the magic of a subquery comes in. Since this query returns a list of values, you can simply use it in the original query as the SQL IN statement. It looks like this:

select * from employee
where employee_number in (select distinct saleman_employee_number from customer where saleman_employee_number is not null)

And, that’s it. You have now used the SQL IN statement with Subquery.

Let me know what you think by commenting or sharing on twitter, facebook, google+, etc.

If you enjoy the video, please give it a like, comment, or subscribe to my channel.

You can visit me at any of the following:

SQL Training Online: https://www.sqltrainingonline.com

Twitter: http://www.twitter.com/sql_by_joey

Google+: https://plus.google.com/#100925239624117719658/posts

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October 26, 2012 by Joey Blue Leave a Comment

SQL Variable Declaration – How to Declare a Variable in SQL Server

Video: SQL Variable Declaration – How to Declare a Variable in SQL Server – SQL Training Online
In this video, I show you how to Declare a Variable in SQL Server using SQL Server Management Studio.

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How do you declare a variable in SQL Server?

There are 3 things to know when declaring a SQL Server variable.

SQL Server Variables start with ‘@’.
You must use the DECLARE keyword.
You can use SET or SELECT to put data into the variable.

So, know I want to show you an example.

SQL Variable Declaration

The first step is to create an empty variable. You do that with the DECLARE statement like this:

DECLARE @my_int int;

Here I have declared a variable that can store an integer.

The next step is to put something into the variable. You can do that by using the SET or the SELECT statements.

SET @my_int=3;

In that example, the variable will store the integer 3.

Then, the last step is to print out what is in the variable. You can do that with the SQL SELECT.

SELECT @my_int;

And that is it. But, you have to run the whole thing together for it to work.

DECLARE @my_int int;
SET @my_int=3;
SELECT @my_int;

Now, you can also change what is stored in the variable. In the following example, I change the value inside of the @my_int variable from 3 to 3+1 which equals 4.

DECLARE @my_int int;
SET @my_int=3;
SET @my_int=@my_int + 1;
SELECT @my_int;

And, the last thing I want to do is show you how to use the SELECT to populate the variable instead of the SET.

DECLARE @my_int int;
SELECT @my_int=3;
SET @my_int=@my_int + 1;
SELECT @my_int;

That’s it. That is how you do a SQL Variable Declaration.

Let me know what you think by commenting or sharing on twitter, facebook, google+, etc.

If you enjoy the video, please give it a like, comment, or subscribe to my channel.

You can visit me at any of the following:

SQL Training Online: https://www.sqltrainingonline.com

Twitter: http://www.twitter.com/sql_by_joey

Google+: https://plus.google.com/#100925239624117719658/posts

LinkedIn: http://www.linkedin.com/in/joeyblue

Facebook: http://www.facebook.com/sqltrainingonline