About the Series …
This article continues the series, MS Access for the Business Environment. The primary focus of this series is an examination of business uses for the MS Access relational database management system. The series is designed to provide guidance in the practical application of data and database concepts to meet specific needs in the business world. While the majority of the procedures I demonstrate will be undertaken with MS Access 2002, many of the concepts that we expose in the series will apply to other versions of MS Access.
For more information on the series, as well as the hardware / software requirements to prepare for the tutorials we will undertake, please see Tutorial 1: Create a Calculated Field with the Expression Builder.
Introduction to this Tutorial
As many of us know from general business experience, the crosstab format is popular for data presentation among a wide range of information consumers. The typical objective of a crosstab query is to display summarized values from one field in a table, grouping them by one set of dimensions listed down the left side of the dataset and another set of dimensions listed across the top of the dataset.
While we can certainly rely upon the Crosstab Query Wizard to guide us through the creation of a basic crosstab query, the objective of this tutorial is to explore the creation and use of an SQL query to render a result dataset in a crosstab format. To accomplish this, we will introduce the TRANSFORM statement, and then use it in a multiple-step practice example to become familiar with its operation.
Along with an introduction to the TRANSFORM statement, this lesson will include:
- an examination of the syntax surrounding the use of the statement;
- illustrative examples of the use of the statement in a multi-step practice exercise that constructs a crosstab query;
- a brief discussion of the results datasets that we obtain in each step of the practice example.
We will use the Northwind sample database that installs with MS Access for purposes of this lesson, in an attempt to make the lesson useful to virtually anyone who has access to MS Access.
The TRANSFORM Statement
When we summarize data using a crosstab query, we select and position values, from specified fields or expressions, as column headings to obtain a results dataset that is presented in a more compact format than what we would see with a standard SELECT query. The TRANSFORM statement puts into play an aggregate function within a SELECT statement to bring about the generation of the crosstab query.
We will examine the syntax of the TRANSFORM statement to gain an understanding of its components, and then we will build a crosstab query. Starting with a basic SELECT statement, we will add the TRANSFORM statement in multiple steps to illustrate its use in reaching our end objective.
Discussion
As we noted in the introduction, a crosstab query usually displays summarized values, or aggregates, from one field in a table. The crosstab query generates a table that has both a row header and a column header; it groups the aggregates by a set of dimensions on the left-axis, and another set of dimensions listed across the top of the dataset. The resulting aggregate data is therefore listed in the table under a cross-reference between these row and column headers – an intersect point, in other words. This is a convenient way to display data in a compact, summarized format.
The available aggregate functions, which are used to calculate statistical information when applied to a specified numeric field in the SELECT query, include the functions shown in Table 1.
FUNCTION
OUTPUT
AVG
Calculates the arithmetic mean of a specified set of values
COUNT
Calculates the number of records returned by a query
MAX
Returns the maximum of a set of specified values or a table
MIN
Return the minimum of a set of specified values or a table
STDEV
Estimates standard deviation, a measure of how widely values are dispersed from a mean, based upon a sample of a population
STDEVP
Estimates standard deviation, a measure of how widely values are dispersed from a mean, based upon a population
SUM
Returns the sum of a set of values contained in a specified field
VAR
Returns estimates of the variance for a specified population sample
VARP
Returns estimates of the variance for a specified population
Table 1: Aggregate Functions
We most often see the COUNT() and SUM() functions used in cases where the SELECT query is being subjected to TRANSFORM.
When we use the TRANSFORM statement to convert a standard SELECT query to a crosstab query, we specify the fields we want to display as row headings, together with using a GROUP BY clause that specifies row groupings. We specify the column headings with a PIVOT operator, and have the option of using an IN clause to limit those headings to fixed values. Finally, we can include other clauses, such as WHERE, as in any SELECT query, to specify additional selection or sorting criteria.
We will see how a simple SELECT statement constructed for the Northwind database can be converted to a crosstab query, and how we can leverage the components of the TRANSFORM statement, in the steps of the practice example that follow.
Syntax
Syntactically, the TRANSFORM statement is used in the following basic manner:
PARAMETERS [prompt] type;
TRANSFORM aggregate function
SELECT select statement
PIVOT pivotfield [IN (value1 [, value2 [, …] ] ) ]The TRANSFORM statement contains the components described in Table 2:
Component
Description
PARAMETERS declaration (optional)
The optional PARAMETERS declaration allows us to specify a prompt that we wish to appear at run time, as well as to specify the data type of the prompt input.
aggregate function
An SQL aggregate function that operates on the selected numerical field values. The subtotals generated appear under column headings in the result dataset.
select statement
A SELECT statement.
The SELECT statement may contain the following additional clauses (of the following, only GROUP BY is required):
- WHERE (optional) – Restricts the results dataset to a specified subset
- GROUP BY (mandatory) – Enforces aggregation within row headings as selected in the specified fields list of the SELECT statement
- ORDER BY (optional) – specifies the ordering of the row headings
pivotfield
Defines the field or expression used in creating column headings in the query’s result dataset.
IN Predicate (value1, value2, etc.)
(optional)
Value1, value2, etc. comprise a comma-separated list of desired column headings. The optional IN predicate, followed by our specified list of column headings, provides us the flexibility to show, or otherwise control order and appearance of, specific fields.
Table 2: Transform Statement Components
AS we have already seen, the TRANSFORM statement needs to come before the SELECT query in the SQL string. The SELECT statement specifies the fields used as row headings, and contains a GROUP BY clause that specifies row grouping. (The SELECT query can be quite sophisticated; we can add subqueries, for example, as predicates in the WHERE clause within our crosstab queries.
The pivotfield values appear as column headings in the query’s result dataset. A common example we see in the business world might be pivoting expense values on the month of the posting to generate twelve columns in the crosstab’s result dataset – one for each month of the year (assuming that transactions had occurred on the specified expense accounts during each of the months). To carry this simple illustration a step further, we could easily restrict the results to only show the first quarter of the year by specifying months 1, 2, and 3, in an optional IN predicate. This would constrain the pivotfield to generate headings solely from the fixed values of the first three months of the year.
I like to base whole reports off crosstab queries, in many instances. In such a scenario, little more than formatting need be accomplished within the reporting application. This can mean that we leave the “heavy lifting” involved in the report to the db engine, which is often the more optimal place for this to be handled.
Let’s move into a hands-on illustration to reinforce our understanding of crosstab queries, using the Northwind sample database that accompanies a typical installation of MS Access.
