Microsoft Access and MySQL language and VB appilications

1. 9/8/2014 Access Tips: Access and SQL Part 4: Building Select Queries On-the-Fly
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Access & SQL 4
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C ontinue the
tutorial:
Part 1:
Introduction
Part 2: VBA &
SQL
Part 3: Examples
Part 4: Building
Queries
Part 5: More
Instant Queries
Part 6: Dynamic
Reports
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Coding Note
Sometimes lines of code
can be very long! Where I
have had to break a line of
code to fit it on the page I
have used the symbol
to indicate that the line
below is a continuation and
that the code should be
written as a single line
<Blah blah blah
blah>
should be written:
<Blah blah blah blah>
Access and SQL
Part 4: Building Queries "On the Fly"
This tutorial is about using SQL to build queries "on the fly" (or as we propellorheads
say: "at run-time"). This means that you build the queries when you need them,
rather than trying to anticipate the user's needs and preparing them in advance.
The tutorial will deal with regular "select" queries, ones which filter the source data
and show the result to the user (as opposed to "action" queries which manipulate the
data in some way).
You can download a copy of the database used in this tutorial. It contains completed
examples of the forms, queries and code described in the tutorial. Follow the link at
the bottom of this page. The database contains a table listing the details of the staff of
a fictional multinational company. It contains the sort of personal details you might
expect such as FirstName, LastName, BirthDate and Gender together with business
details such as Office, Department, JobTitle and Email.
This tutorial is in two parts. This first part will show you how to create a fully working
multi-purpose query. The second part will explain how to add some refinements to
create a really professional query tool. You will find a link to the second part of the
tutorial at the bottom of this page.
Why Build Queries on the Fly?
Like many Access developers, I tend to create very few stored queries. Instead I try
to create a small number of general purpose queries that can be changed on demand
to suit the user's requirements. This allows my databases to be much more flexible
and I don't have to guess what the users might want to know. I can then use
switchboards and dialog boxes to gather the information from the user which is used
to create the query's SQL statement. The user is really building a query themselves
but they don't need to know anything about Access to do it.
Another important reason for working this way is that inquisitive (or careless!) users
might delete or change stored queries, and not know how to replace them.
Building a Multi-Purpose Query
The Plan...
The aim of this project is to create a single stored query whose criteria can be
changed to suit the user's requirements. The query will be opened when the user
clicks a button on a dialog box. The dialog box will also be used to gather the criteria
from the user.
The Stored Query
You need a query that can be used as the basis of our multi-purpose query. Its design
is completely irrelevant because it is going to be changed each time it is used, but
Access doesn't let you create an "empty" query so you'll have to make something up
(anything will do).
Ask Access for a new query in design view, add a table, put a field into the grid and
then close and save the query. Give it a sensible name - in this example I am calling
the query qryStaffListQuery.
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2. 9/8/2014 Access Tips: Access and SQL Part 4: Building Select Queries On-the-Fly
The Dialog Box
I have decided that my users will probably want to query on three different criteria:
Office, Department and Gender. So, the first step is to build a dialog box to help the
users specify their criteria. It's up to you what your dialog box looks like and what it
contains. I have chosen to use combo boxes showing lists of all the possible criteria
choices. Future tutorials in this series will show examples of other design methods.
If you are not experienced in building dialog forms like this one, take a look at my
tutorial C ustomizing Access Parameter Queries which contains detailed step-by-step
instructions.
Point at the labels next to the combo boxes in the illustration below to see what is
contained on their lists...
So, now we have the required components of the multi-purpose query. The next step
is to write the code to make them work together.
Writing the VBA and SQL Code
When the user clicks the dialog's OK button several things must happen:
Gather the user's choices from the combo boxes and write them into an
SQL statement.
Apply the SQL statement to the stored query.
Open the stored query.
C lose the dialog box.
The code to carry out these operations will run on the On Click event of the OK
button.
In form design view right-click the OK button and choose Properties to open its
properties window and locate On Click on the Events tab. C lick in the white bar then
click the Build button: . In the Choose Builder dialog select Code Builder and
click OK. You are now ready to write the code...
Declare and Fill the Variables
The first few lines of code establish contact with the database, telling Access that we
are referring to the current database (i.e. the one containing the code) and identifying
the query that we are going to work on. In addition, a string (i.e. text) variable is
declared, which I have called strSQL. It will hold the SQL statement that will be applied
to the query:
Dim db As DAO.Database
Dim qdf As DAO.QueryDef
Dim strSQL As String
Set db = CurrentDb
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3. 9/8/2014 Access Tips: Access and SQL Part 4: Building Select Queries On-the-Fly
Set qdf = db.QueryDefs("qryStaffListQuery”)
NOTE: I am using DAO language here because I think it is simpler
for this sort of work. DAO is the default for Access 97, but the default
for Access 2000/2002 is ADO. Access 97 users need do nothing (you
can omit the "DAO." bits if you want but it doesn't really matter) but
Access 2000/2002 users need to set a reference to DAO so that their
database understands the code. In the Visual Basic Editor go to
Tools > References. In the dialog box scroll down to "Microsoft
DAO 3.x Object Library" (where x is the highest number if you have
more than one) and put a tick in the box. C lick OK to set the
reference. You only need to do this once for the database and any
code you put in it will be able to use the reference.
Build the SQL Statement
Next comes a VBA statement which places a text string into the strSQL variable. There
was a detailed explanation of how I like to write my VBA/SQL and the rules you need
to know in the second tutorial in this series: Access and SQL Part 2: Putting VBA and
SQL Together.
This code combines SQL keywords and clauses into which have been placed
references to the combo boxes that contain the user's criteria choices:
strSQL = "SELECT tblStaff.* ” & _
"FROM tblStaff ” & _
"WHERE tblStaff.Office='" & Me.cboOffice.Value & "’ ” & _
"AND tblStaff.Department='" & Me.cboDepartment.Value & "’ ” & _
"AND tblStaff.Gender='" & Me.cboGender.Value & "’ ” & _
"ORDER BY tblStaff.LastName,tblStaff.FirstName;”
Although it doesn't seem to make a readable SQL statement as it is, when Access
reads the code and substitutes, for example, Me.cboOffice.Value with London a
sensible SQL statement results.
Here is what your code should look like so far [click the thumbnail to see a full-sized
image]:
Test the Code
Now is a good time to test the code you have written so far and there are a couple of
ways you can do this. You can "print" the SQL to the Immediate Window or you can
display it in a message box (or you can do both!). Here's what to do:
Using the Immediate Window:
In Access 97 the Immediate Window is called the Immediate (lower) pane of the
Debug Window. In all versions the window can be displayed by pressing Ctrl+G from
the VBA code window. First, add the following line of code before the End Sub line of
your cmdOK_Click procedure:
Debug.Print strSQL
Using a Message Box:
Add the following line of code before the End Sub line of your cmdOK_Click procedure:
MsgBox strSQL
Now you are ready to run a test. Switch to Access and open your dialog box in Form
View (now is a good time to save the form!). Make some choices from the combo
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4. 9/8/2014 Access Tips: Access and SQL Part 4: Building Select Queries On-the-Fly
boxes and click the OK button...
If you chose to use a message box it will open displaying the SQL string that your
code created from the choices in the dialog box. Read the SQL statement to check that
it makes sense:
If you chose to use the Immediate Window, switch to the VBA code window and press
Ctrl+G to open the Immediate Window where the SQL string will be displayed (it is
written in a single line):
The message box method is quick and, because I am familiar with SQL, it is my
preferred method. Using the Immediate Window has the advantage that you can
select and copy from it the SQL statement that your code generated and paste it into a
query to test it. To do this first select and copy the SQL statement then return to the
Access database window and choose Create query in design view. C lose the
Show Table box and open the SQL window by choosing View > SQL View. Delete
any entry that is already there (it usually shows SELECT;) and paste in your SQL
statement. Run the query and check the result.
If you see an error message when you try to run your SQL you can trace the error
and correct it. Read the section on "Debugging Your SQL C ode" in Part 2 of this series.
When you are satisfied that this part of your code is working properly, you can
remove the line(s) Debug.Print strSQL or MsgBox strSQL.
Update and Open the Query
All that remains is to apply the SQL statement to the stored query that you saved
earlier, and to close the dialog box.
Enter the line:
qdf.SQL = strSQL
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5. 9/8/2014 Access Tips: Access and SQL Part 4: Building Select Queries On-the-Fly
to apply your new SQL statement to the stored query. There is no need to give a
command to save this change to the query because this happens automatically. Next
add the lines:
DoCmd.OpenQuery "qryStaffListQuery"
DoCmd.Close acForm, Me.Name
to open the query displaying the results of the user's criteria choices, and to close the
dialog. Finally add the lines:
Set qdf = Nothing
Set db = Nothing
which empty the variables used to identify the query and the database. All variables
normally lose their values when a procedure finishes, and the memory that was
allocated to them is freed. But sometimes Access forgets to clear "object" variables
(those with which you have to use the "Set" keyword) so it is good coding practice to
clear them manually like this, just to make sure. Your finished code should look like
this [click the thumbnail to see a full-sized image]:
Return to the Access database window and save the dialog box form (to save your
code changes).
Job Done!
Your Multi-Purpose Query is now ready to run. Remember that the query should be
run from the dialog box. If you open the stored query it will display records using the
same criteria as the last time it was run.
The Multi-Purpose Query will work fine as it is, but you can make it even more user-friendly
with the addition of a few refinements. The second part of this tutorial shows
you how to turn your multi-purpose query into a really professional tool.
Go to the next part of the tutorial >>>
^ top
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