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Using Derived Tables to Simplify the SQL Server Query Process

MSSQLTips author Greg Robidoux By:   |   Read Comments (9)   |   Related Tips: More > Query Optimization

Problem
Sometimes querying data is not that simple and there may be the need to create temporary tables or views to predefine how the data should look prior to its final output.  Unfortunately there are problems with both of these approaches if you are trying to query data on the fly. 

With the temporary tables approach you need to have multiple steps in your process, first to create the temporary table, then to populate the temporary table, then to select data from the temporary table and lastly cleanup of the temporary table.

With the view approach you need to predefine how this data will look, create the view and then use the view in your query.  Granted if this is something that you would be doing over and over again this might make sense to just create a view, but let's look at a totally different approach.

Solution
With SQL Server you have the ability to create derived tables on the fly and then use these derived tables within your query.  In concept this is similar to creating a temporary table and then using the temporary table in your query, but the approach is much simpler, because it can all be done in one step.

Let's take a look at an example where we query the Northwind database to try to find out how many customers fall into various categories based on sales.  The categories that we have predefined are as follows:

  • Total Sales between 0 and 5,000 = Micro
  • Total Sales between 5,001 and 10,000 = Small
  • Total Sales between 10,001 and 15,000 = Medium
  • Total Sales between 15,001 and 20,000 = Large
  • Total Sales > 20,000 = Very Large

There are several ways that this data can be pulled, but let's look at an approach using a derived table.

The first step is to find out the total sales by each customer, which can be done with the following statement.

SELECT   o.CustomerID,
         
SUM(UnitPrice QuantityAS TotalSales
FROM     [Order Details] AS od
         
INNER JOIN Orders AS o
           
ON od.OrderID o.OrderID
GROUP BY o.CustomerID

This is a partial list of the output:

CustomerID TotalSales
ALFKI  4596.2000
ANATR 1402.9500
ANTON 7515.3500
...  
WOLZA 3531.9500

The next step is to classify the TotalSales value into the OrderGroups that were specified above:

SELECT   o.CustomerID,
         
SUM(UnitPrice QuantityAS TotalSales,
         
CASE 
           
WHEN SUM(UnitPrice Quantity
               BETWEEN 
AND 5000 THEN 'Micro'
           
WHEN SUM(UnitPrice Quantity
               BETWEEN 
5001 AND 10000 THEN 'Small'
           
WHEN SUM(UnitPrice Quantity
               BETWEEN 
10001 AND 15000 THEN 'Medium'
           
WHEN SUM(UnitPrice Quantity
               BETWEEN 
15001 AND 20000 THEN 'Large'
           
WHEN SUM(UnitPrice Quantity
               > 
20000 THEN 'Very Large'
         
END AS OrderGroup
FROM     [Order Details] AS od
         
INNER JOIN Orders AS 
          
ON od.OrderID o.OrderID
GROUP BY o.CustomerID

This is a partial list of the output:

CustomerID TotalSales OrderGroup
ALFKI  4596.2000 Micro
ANATR 1402.9500 Micro
ANTON 7515.3500 Small
...    
WOLZA 3531.9500 Micro

The next step is to figure out how many customers fit into each of these groups and this is where the derived table comes into play.  Take a look at the following query which uses a derived table called OG.  What we are doing here is using the same query from the step above, but calling this derived table OG. Then we are selecting data from this derived table for our final output just like we would with any other query.  All of the columns that are created in the derived table are now available for our final query.

SELECT   OG.OrderGroup,
         
COUNT(OG.OrderGroupAS OrderGroupCount
FROM     (SELECT   o.CustomerID,
                   
SUM(UnitPrice QuantityAS TotalSales,
                   
CASE 
                     
WHEN SUM(UnitPrice Quantity
                       BETWEEN 
AND 5000 THEN 'Micro'
                     
WHEN SUM(UnitPrice Quantity
                       BETWEEN 
5001 AND 10000 THEN 'Small'
                     
WHEN SUM(UnitPrice Quantity
                       BETWEEN 
10001 AND 15000 THEN 'Medium'
                     
WHEN SUM(UnitPrice Quantity
                       BETWEEN 
15001 AND 20000 THEN 'Large'
                     
WHEN SUM(UnitPrice Quantity
                       > 
20000 THEN 'Very Large'
                   
END AS OrderGroup
          
FROM     [Order Details] AS od
                   
INNER JOIN Orders AS o
                     
ON od.OrderID o.OrderID
          
GROUP BY o.CustomerIDAS OG
GROUP BY OG.OrderGroup

This is the complete list of the output from the above query.

OrderGroup OrderGroupCount
Large 10
Medium 11
Micro 33
Small 15
Very Large 20

 

Next Steps

  • Next time you run into a challenge of whether to create a temporary table or a view to produce the desired query take a look at using a derived table instead
  • Experiment with using derived tables, views and temporary tables to see what yields better performance results. There are several articles on the internet that have shown that using a derived table is faster then temporary tables, but try it for yourself.
  • Take a look at Common Table Expressions in SQL Server 2005


Last Update: 8/16/2006


About the author
MSSQLTips author Greg Robidoux
Greg Robidoux is the President of Edgewood Solutions and a co-founder of MSSQLTips.com.

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Comments and Feedback:
Tuesday, December 21, 2010 - 12:33:41 PM - Jackie Bird Read The Tip

Tried your derived table example - but when you have multiple fields to pull you have to include all of them in the group by clause - moreover the AS for the inner group by doesn't work - it throws an error and when i remove it the sql executes


Tuesday, December 21, 2010 - 1:31:54 PM - Greg Robidoux Read The Tip

Hi Jackie, what version of SQL Server are you using?

Since all of the data is GROUPed you will need to include any column that is not a summary such as AVG, SUM, COUNT, etc... in the GROUP BY clause.

Also, I am not sure why the AS for the derived table did not work.  This is not really necessary.  I tried this both with and without the AS and it worked fine both ways.


Tuesday, June 19, 2012 - 7:37:13 AM - Sulthanul Arifeen Read The Tip

Derived tables only helpful for small amount data tranction. For huge data transaction with calculation, temp table is better option.  My suggestion is to use temporary table whenever possible only keeping the locking issue under consideration.


Sunday, March 03, 2013 - 11:52:24 AM - Frank Read The Tip

Im am trying to create a sql query that will return the following (1)  from the table snippet (2)  I am using this (3):

(1)

EmdDescription

10D110 - CHEST PAIN - D1 Severe Respiratory distress

10D210 - CHEST PAIN - D2 Not alert

10D310 - CHEST PAIN - D3 Clammy

 

(2)

EmdDescription

10D110 - CHEST PAIN - D1 Severe Respiratory distress

10D210 - CHEST PAIN - D2 Not alert

10D310 - CHEST PAIN - D3 Clammy

11A111 - CHOKING - A1 Not choking now

11D111 - CHOKING - D1 Not alert

11D211 - CHOKING - D2 Abnormal breathing

11011 - CHOKING - E1 Choking verified or ineffective breathing

12A112 - CONVULSIONS SEIZURES - A1 Not seizing now regularly

12B112 - CONVULSIONS SEIZURES - B1 Breathing regular not verified < 35 yrs

 

SELECT *

FROM EmdCodes

WHERE Description ="%CHESTPAIN%";

 

Any help is appreaciated.

 

 

 

 


Wednesday, March 06, 2013 - 2:50:38 PM - George Read The Tip

Any thoughts on whether using a Common Table Expression is better than a derived table?


Monday, April 01, 2013 - 11:01:43 AM - RHG Read The Tip

Very helpful, thanks a lot


Monday, April 01, 2013 - 3:35:27 PM - Greg Robidoux Read The Tip

@Frank - I think you just need a space in your query:

SELECT *

FROM EmdCodes

WHERE Description ="%CHEST PAIN%";


Monday, April 01, 2013 - 3:36:42 PM - Greg Robidoux Read The Tip

@George - not totally sure if a CTE will perform better.  The best option is to test both methods and see what is eaiser to setup and also what performs better.


Wednesday, December 11, 2013 - 10:48:26 AM - BuPhilOh Read The Tip

Simple, elegant, informative.  Just what I needed as a newbie bumping into the problem of how to do calculations based on the results of other calculations.  Thank you, Greg!  Now to see if I can put it to work....



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