Tag: from

Anatomy of a Select Statement Part 3

Welcome to part 3 of this series.  The first part covered the order of execution of the commands, while the second session went in depth about the SELECT command. This post will cover the FROM clause.  Most people would think that you will list tables in the FROM clause.  Yes, that is correct, however you can do so much more.

What can be listed in the FROM

This first step is to list what can you use in the FROM clause. The list includes the below list.

  • Table
  • View
  • Temporary Table
  • Table Variable
  • Derived table
  • Table valued functions

We will cover each of these.

Table and View

Tables and views are probably the most common objects used in the FROM, although I really don’t have any exact statistics on that. Below is an example is a simple query.

SELECT TOP (1000) [ProductID]
,[Name]
,[ProductNumber]
,[MakeFlag]
,[FinishedGoodsFlag]
,[Color]
,[SafetyStockLevel]
,[ReorderPoint]
,[StandardCost]
,[ListPrice]
FROM [AdventureWorks2014].[Production].[Product]

In the FROM clause you will see the table or view.  Notice that not only is the table name listed, but the database and schema are also specified.  Those two are not require, but I might not be a bad idea to start listing the schema as well as the table name.

By looking at this statement, we can only speculate that the object listed is a table.  If you use a view instead, they will look that same.  Really the only way to tell if the object is a table or view is to simply know what type of object it is.  You can look in the objects explorer in SSMS and look for the object.  Sometimes using the naming convention can also be used to make this determination.

Temporary Table

Temporary tables are created in the TempDB system database.  A temporary table can be identified by the start of the table name.  The name will start with either # or ##.

Phil Factor on Red-Gate’s web site gives a great description of when we should use temporary tables. Here is the link: RedGate

“They are used most often to provide workspace for the intermediate results when processing data within a batch or procedure. “

When a temporary table starts with # it is a “local” table.  This means that the only process that can see and use the temporary table is the one that created it. While global temporary tables will start with ##, they are not as secure because they can be used by other processes.

Since local temporary tables are used only by the process that created it, it will automatically be dropped when the connect that created it ends.  Global temporary tables could persist a bit longer because it will remain until all processe that are using it are terminated.  This post isn’t an in depth discuss about temporary tables, you can to here to get more information: TempTables.

Prior to using a temporary table, it must be created.  You can use the CREATE TABLE statement or use SELECT…INTO.

–Create the table
CREATE TABLE #test
(ProductName VARCHAR(50)
, Price MONEY)

–Populate the table
INSERT #test
SELECT Name
, ListPrice
FROM Production.Product

–Use the table
SELECT *
FROM #test

In the code above you will see three distinct sections.  The first is to create the temporary table, populating the table is the second and the third and last step is the use the table.

If you attempt to query the table and it is out of scope you will get the following error.

You receive this error because with a local temporary table, it is only available to the session that created it.  That is not the case with a global temporary table, one that starts with ## rather than # for a local table.

However if you change from a local to a global temporary table, you will not receive the above error.  You will actually get the results you are hoping for. If you look at the image below, you will see that the table was create with SPID 51 and used with SPID 60.  Also note that even though it is a valid table, SSMS is still indicating that it is an invalid object in SPID 60.  Of course that is not the case, it works just fine.

As you can see, temporary tables are used in pretty much the same way as user tables in a database. You just need to be aware of the scope of the table, is it local or global.

Table Variable

A table variable follows the same steps for creation and use.  Instead of using the CREATE TABLE statement, when using a table variable, you use the DECLARE statement to declare the variable as a TABLE datatype.

The scope of a table variable is the batch.  If you create a temporary table you can use it for the duration of the session.  While the duration of a table variable is the batch itself.

There are some differences on how SQL Server processes temporary tables and table variables.  That is not the purpose of this post.  You can go here to find more information regarding the differences.

Derived table

When I teach the Microsoft SQL Server certification course on TSQL, a derived table is in the same chapter as subqueries. A derived table is really not much more than a SELECT statement in the FROM clause instead of a table name.  As you can see from the code below, the derived table is highlighted.  The derived table has an alias of “v” and is enclosed in parenthesis.

Table Valued Function

Like temporary tables and table variables, before using a table valued function(TVF) the function must be created.

The code below shows how to create the function.  This function returns a table. This is from Microsoft’s web site. This will work on the AdventureWorks2014 database.

CREATE FUNCTION ProductsCostingMoreThan(@cost money)
RETURNS TABLE 
AS 
RETURN 
SELECT ProductID, ListPrice
FROM Production.Product
WHERE ListPrice > @cost

Once the function is created you can see in SSMS under Programmability.

One of the advantages of a TVF is that they can accept parameters.  In the code above, the parameter is for the list price. In the code below you will that the TVF is used just a user table might be.  Also notice the parameter being passed into the function.

In this case, we are looking for any products that cost more than $80.50 and then joining it on the Product table.

Each of these objects has performance advantages and disadvantages.  That is not the purpose of this post, however it is something that must be considered when determining the appropriate solution.

In the Part 4 on this topic, I will cover JOINS.

Thank for visiting my blog.  I hope you learned something.

 

 

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

LEFT RIGHT LEN
UPPER LOWER REVERSE
TRIM GETDATE DATEDIFF
UPPER LOWER TRY_CONVERT

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.

TOP

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.

$ROWGUID and $IDENTITY

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.

DISTINCT

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

 

 

 

 

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]
,[Name]
,[ProductNumber]
,[MakeFlag]
,[FinishedGoodsFlag]
,[Color]
,ListPrice
,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!!!