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!!!