Anatomy of a Select Statement Part 2

The Select statement is one of 4 Data Manipulation Language(DML), the others being INSERT, UPDATE and DELETE. As expected the SELECT statement is used to extract data from a relational database such as SQL Server.  This statement works in most database systems.  Although, there may be some differences in the different products. These differences are not the focus of this post, we will concentrate on the use of the SELECT statement in Microsoft SQL Server.  Over the years the SELECT statement has not changed much as versions of SQL Server are released, making this one of the statements that will work in older versions as well as the new ones. This is part 2 of the series.

The SELECT clause is just one of a few clauses in the SELECT statement.  The others are FROM, WHERE, GROUP BY, HAVING and ORDER BY.  In part 1 I discussed the order these clauses are processed.  As a reminder, the SELECT clause is either last or second to the last in the processing order.  If there is an ORDER BY, the SELECT is processed just before the ORDER BY.  If there is not an ORDER BY, then the SELECT is the last clause processed.  The focus of this post is just the SELECT clause.

The SELECT clause can easily be described as the place where you identify what columns you want to be included in the result set.  These columns can be columns from the table or view, an expression or the results of a scalar function.  In addition to what columns you want included in your result set, the SELECT clause is also where the order in which those columns appear in the result set is determined.

When deciding what columns from the tables to be included in the SELECT clause, it is important to only return the columns that are needed.  Avoid using “SELECT * FROM….”.  Sometimes it is simply easier to use SELECT *, however there are consequences to using it, mostly the potential impact on the performance of the query.   When listing the columns they need to separated by a comma.  Below you will find an example.  The “[” and “]” are used when the column name contains a space.  You will also see them if you right click on a table and click “Select Top x rows” in SSMS.

In addition to actual column names you can also use calculated columns in the SELECT clause. However, when this is done the column name in the result set will be (No Column Name) unless a column alias is used.

Result Set without a column alias:

The example below show how a column alias can be used.  The “AS” keyword isn’t required.  The single quotes surrounding “New Price” are also not required as long as the alias name does not include spaces.  Personally I also like to include them regardless if there is a space or not.  There really aren’t a lot of requirements for column aliases although I would try to avoid using keyword and actually column names.

Scalar Functions

Scalar functions can also be used in the SELECT clause.  These functions can be system supplies functions or user defined functions. Really the only requirement is that it must be a scalar function, not a table valued function. Just a reminder, a Scalar function is a function that returns a single value.  So when one is included in a SELECT statement, it will run the same number of times as there are rows returned.

Here are some examples of a few native SQL Server built-in functions. More information can be found here: More Info on Functions


When using these functions, many times the arguments will be populated with column names.  Although that is in no way a requirement.

Below you will find an example of using two Built-In functions. One, UPPER, requires an argument to be used while the second, GETDATE(), does not.  However, both would need a column alias.  This would be the same if using User Defined Functions(UDF).

Example using a UDF.  In this case it is the ufnGetSalesOrderStatusText UDF in the AdventureWorks database.  It accepts TINYINT value as an input parameter and will return the text description of the status.

If you run the above query without the TOP clause, you will see that this query returns over 31,000 records.  This means that the UDF will execute over 31,000 times. This is important to know in case there are performance issues.


We have discuss adding columns, column aliases as well as the use of scalar functions in the SELECT clause. Now lets talk about the TOP key word. This appears immediately after the SELECT key word.  This is used to limit the number of rows to a specific number, TOP(10) or percent of rows, TOP 10 PERCENT.  The TOP key word is usually used in conjunction with the ORDER BY clause.  If no ORDER BY clause is included, then SQL Server will per Microsoft,  “it returns the first N number of rows in an undefined order”.

In the example below, only 10 rows are return based on the oldest 10 records when sorted by the OrderData.  Remember, the default sort order is ascending.  Notice that there are not parenthesis around the number 10.  Again, not required at this point, but will be in the future.

Notice above that the OrderDate is the same.  That is because it only returned the top 10 records.  Well what if there are more than 10?  This is where WITH TIES comes in.  If you look at the example below you will see that there are 11 rows.  This is because the 11th row match the value of the 10th row.  WITH TIES will return if there are additional matches with the value in the last row.


These two functions are used with tables that have an Identify column or a column using the ROWGUIDCOL property.  With both $ROWGUID and $IDENTITY you actually don’t need to know the name of the columns.  If you attempt to use either of these on a table that does not have the proper column types will result in an invalid column error.

Notice in the example below, the first column and the last column in the result set are the same.  The $ROWGUID column, the first column, will also assume the column name of the source column as well. $Identify works in the same way.


The last topic in the post is the use of the key word DISTINCT.  DISTINCT is used to eliminate duplicates in the result set.  Per Microsoft, NULLS are considered equal for the DISTINCT keyword.

If you look below you will see that the number of rows returned is 316.  If you add the keyword DISTINCT the number of rows returned drops to 89.  With this query, all duplicates based on Gender and Rate are removed.

During the post we have discussed many of the items that can be included in the SELECT clause. This includes column aliases, user defined functions, built-in functions, TOP and DISTINCTS keywords as well as a few other topics.

Hopefully you leaned something and thank you for visiting my blog. In the next post I will dissect the FROM clause.

Previous parts of the Series

Part 1 – Order of the Clauses in the SELECT statement





Where did it go???

Have you have ever looked for something and expected it to be there…and it wasn’t? If so, you will understand my day today. I have a SQL Server Agent job that uses VBScript in one of it’s job steps and we tried to move it to a SQL Server 2016 server.  It unexpectedly failed because the ActiveX Script job step type is no longer an option.

Below you will find a screenshot from a pre-2016 server.  As you can see, the job step type is there.

Now here is a screenshot of the job step properties from a SQL Server 2016 server.  Notice, NO script job step type.  However, you will see a PowerShell job step type that we can use instead.

Here is the link to Microsoft that describes other features that were removed from SQL Server 2016.

You can query the sysjobsteps table to find the jobs that have these types of job steps.

, s.step_id
, s.step_name
, s.subsystem
FROM dbo.sysjobsteps s
         INNER JOIN sysjobs j
                 ON s.job_id = j.job_id

I have been telling my students for a number of years that as DBAs we will need to learn PowerShell someday. That day actually arrived a few years ago.  This is a great example of why we should know PowerShell.

I really try to keep up on the features that are removed from versions of SQL Server, however this one slipped through the cracks and I missed it.  I just really never expected it to be removed, although I do understand why MS did remove it.


Max Degree of Parallelism vs Cost Threshold for Parallelism

The title sounds like something that you would see for a boxing match.  However, in reality it is a misleading title.  The Max Degree of Parallelism and the Cost Threshold for Parallelism SQL Server settings actually work more together than they do against each other.

These two settings actually define the how many and the when in regards to parallel execution plans.  The Max Degree of Parallelism(MDop) simply defines the number of processors\cores that SQL Server will use when the optimizer determines parallelism is needed.  The Cost Threshold for Parallelism is cost threshold of when SQL Server will use parallelism.  The cost is the overall cost the optimizer determines for each query and SQL Server will use parallelism if the cost is above the threshold value.

The recommended settings for MDop is the number of cores not to exceed 8.  However, when setting this, it is important to continue to monitor the system to see if the change has caused an improvement. The default value is 0.

The recommended setting for the Cost Threshold for Parallelism is 25 to 50 and has a default value of 5.  Yes, the default setting is too low.  By changing this, SQL Server will reduce the number of smaller queries that may use parallelism.  Just like the MDop setting, it is important to monitor the server after making a change to see if an improvement is make.  There have been many times where simply changing this setting from the default, I have seen the CPU utilization drop from close to 100% to less than 10%.  This in no way means you will see the same improvement, just what I have seen.

Neither one of these settings require a reboot of the service when changing.  It is also important to keep in mind that if the MDop is set to 1, SQL Server will ignore the Cost Threshold for Parallelism setting.

Kendra Little has a very nice video that explains this more in depth.

MAXDOP of Confusion (Dear SQL DBA Episode 8)


This snippet of code can be used to query server configuration settings

, value
, description
FROM sys.configurations

Thanks for reading!!!!!


Anatomy of a SELECT Statement – Part 1

Well here it is, my first blog post.  In writing this I realized I can talk for hours on a topic and struggle when writing a blog post.  Writing blog posts is much harder than I had thought.  My plan is to start with something simple and work my way into more complex topics.  Luckily there are many great blogs out there in the SQL Server community that I can use as a model of how blogs should be done.

The Select statement is one of 4 Data Manipulation Language(DML), the others being INSERT, UPDATE and DELETE. As expected, the SELECT statement is used to extract data from a relational database such as SQL Server.  This statement works in most database systems, although there may be some differences in different products. These differences are not the focus of this or future posts, we will concentrate on the use of the SELECT statement in Microsoft SQL Server.  Over the years the SELECT statement has not changed much as versions of SQL Server are released, making this one of the statements that will work in older versions as well as the new ones.  This is the first of several that will dissect the SELECT statement and provide information on each.  I will start with the order of the commands in a SELECT statement.

Order of the Commands in the SELECT Statement

The SELECT statement consists of multiple parts. Below you will see the order in which the statements must appear.

If they are not in the proper order, an error will be raised and the query will not be allow to process properly.

Below you will find the error you will see if the statements are not in the proper order:

Msg 156, Level 15, State 1, Line 9

Incorrect syntax near the keyword ‘FROM’.

Understanding this order is vital to help explain why column aliases cannot be referenced in any statements besides the ORDER BY.  Just a quick reminder, column level aliases can be used on all columns, however calculated columns and columns that use functions will not have a column name unless an alias is used.

If you run the statement below you will receive this error:

Msg 207, Level 16, State 1, Line 2

Invalid column name ‘NewPrice’.

SELECT TOP 20 [ProductID]
,ListPrice * 2 AS ‘NewPrice’
FROM [AdventureWorks2014].[Production].[Product]
WHERE NewPrice > 100

The reason the error happens is because when the WHERE statement runs, the alias doesn’t exist yet.  With the above statement, the SELECT statement will actually run after the WHERE, therefore causing an error.

Well, how do I make it work you may ask?  The small snippet below shows how.  Rather than reference the alias in the WHERE clause, you will need to repeat the formula.  In this case the formula is the ListPrice Column times 2.

WHERE ListPrice * 2 > 100

However, you can reference the alias in the ORDER BY clause because it processes after the SELECT clause and the alias exists when the ORDER BY processes.

Hopefully you have made it this far!!!  As you can see in some situations, understanding the processing order of the commands in a SELECT statement is important.  I will cover the SELECT command in my next post.

Thanks for visiting my blog!!!

Welcome to my blog

I really enjoy teaching SQL Server and I am hoping my blog will allow me to share knowledge with more people.  I hope you enjoy and more importantly, you learn SQL Server and how great it is!!!