Using Derived Tables to Simplify the SQL Server Query Process

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 * Quantity) AS 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:

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

 SELECT   o.CustomerID, 
         SUM(UnitPrice * Quantity) AS TotalSales, 
         CASE  
           WHEN SUM(UnitPrice * Quantity)  
               BETWEEN 0 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.CustomerID 

This is a partial list of the output:

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.OrderGroup) AS OrderGroupCount 
FROM     (SELECT   o.CustomerID, 
                   SUM(UnitPrice * Quantity) AS TotalSales, 
                   CASE  
                     WHEN SUM(UnitPrice * Quantity)  
                       BETWEEN 0 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.CustomerID) AS OG 
GROUP BY OG.OrderGroup 

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

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