Low Code Cascading Combo Boxes With TempVars

Cascading Combo Boxes are a staple of interfaces for Relational Database Applications. Cascading Combo Boxes are a series of 2 or more Combo Boxes in which each Combo Box is filtered according to a selection made in the previous Combo Box. 

For example, a vendor might use a series of combo boxes to find customers in a specific city by first selecting a State from a combo box bound to a Lookup table of States. That selection filters the City list in a combo box bound to a table of Cities. And that, in turn filters the list of customers to those whose address is found in the city selected in the City combo box.

Some time ago, I decided to see if I could simplify the process by using TempVars as the criteria for those combo boxes and forms. It turns out that there are a couple of advantages to using TempVars instead of more traditional approaches.

Dynamic SQL

One common approach is to dynamically re-write SQL Statements for "downstream" combo boxes to apply the filter selected in the "upstream" combo box. Something like this:

    strSQL = "SELECT CityName FROM tblCustomerAddress 
        WHERE StateName = '" & Me.cboSelectState & "'"
    Me.cboSelect.RowSource = strSQL

Form and Control References in Queries

Another approach is to incorporate a form control reference directly in the query, like this:

       SELECT CityName FROM tblCustomerAddress
        WHERE StateName = Forms!frmCustomers.cboSelectState

In both cases, as long as the form is open, the downstream, city, combo box will properly return the list of cities when it is requeried after a selection in the upstream, state, combo box.

 TempVars  

I decided that I could do better. Instead of dynamic SQL or hard-coded form and control references, I could write queries that filter on TempVars. That gave me two advantages.

First, there's no need to fuss with writing and rewriting SQL dynamically. The rowsource of the downstream combo box, as well as the recordsource of the form, could be written as saved queries that never need to be modified at run time.

Second, because the TempVar can be set or modified as needed, the queries can be opened directly from the Navigation Pane whether the form is open or not. TempVars don't lose their values until some event in the Relational Database Application changes them. And that can make trouble-shooting just a little bit less fussy.

All or One

In addition to the flexibility and ease of use of TempVars, I found an additional tool to make Cascading Combo boxes even more useful. By wrapping the TempVars in an Immediate If  expression, I could select ALL records or only ONE record without having to do any additional coding. You can download a sample Access Relational Database Application using this method here:  Download of Cascading Combo Box From GPC Data.

Design View of Form's RecordSource Query with Iif()

Here are some screenshots of relevant components, starting with the query that populates the form's recordsource. 

SQL View of Form's RecordSource Query with Iif() 

Here's how it works. There are two possibilities. 

The value of TempVar can be 0 or it can be a positive, non-zero long integer. This is set in the AfterUpdate() event of the combo box on the form.

If the TempVar is 0, the Iif() expressions in the criteria select records where 0 = 0. In other words, ALL records will be returned.

If the TempVar is not 0, the Iif() expressions select records where the WorkID is equal to that TempVar value, which will be one record.

All we need to do is set the value of the TempVar in the AfterUpdate() Event and requery the form. 

Private Sub cboSelectWork_AfterUpdate()
   
    With Me
        TempVars.Add Name:="lngWorkID", Value:=Nz(.cboSelectWork, 0)
        .Requery
    End With
 
End Sub


One more trick makes it possible to return ALL records, if the user wants to see them, but we don't bog down the form by automatically doing so.

Here's the SQL from the combo box' rowsource:

UNION Query to Return ALL Records  or ONE Record

 Here's how it works. 
 

Cascading Combo Box Filtered to Work for One Custumer

One Customer, Darleve Sucknere,  has been selected in the upstream combo box for customers. This sets the TempVar that filters the downstream combo box for Work. It shows three projects done for that client, PLUS the option to select All Work for All Customers.

Download and Play with the Demo

 

The Cascading Combo Box with TempVars Demo  shows all of the features called out above, plus a few more enhancements you might like.

Let me know what you think.

Modifying Linked Tables

After two and a half decades of working with Access and answering questions in online forums, I am still surprised from time to time by questions about things I have long taken for granted. Here's a recent case in point.

The Question as Posed

First, the question, as posed, was a bit of a red herring (a lot of them are, as a matter of fact). The Original Poster (OP) wanted to know why his query seemed to "lose" new fields when he tried to add them. After a few back and forth responses with several other members trying to help, it finally turned out that the Access Relational Database Application in question was actually a split design, with a Back End for tables and a Front End for the interface elements including the query that was "losing fields". And from there it became clear what might be the underlying problem. The user was trying to add the new fields in the query and in the linked table in the front end.

The Solution Proposed

Once it became clear that the OP was trying to do this from within the Front End, the solution was much more straightforward. The new fields have to be added in the Back End and only then do they become available in the Front End.

I wish the solutions were all that simple although the detective work it took to get to it was a bit convoluted.

An Illustrated Example

As a follow-up, I decided to sketch out a quick illustration of the steps a successful table modification should follow in a typical Front End-Back End design. I used one of the basic sample data sets I keep for such purposes. It's based on Employees of an organization and the departments to which they are assigned.

Employees-Departments-Department Assignments

The relationship is many-to-many because each employee can be assigned to one or more departments and each department can have one or more employees. In this case, the relationship is intended to be sequential, rather than concurrent, in that employees would be assigned only to one department at a time. For that reason, the "fromDate" is included in the Primary Key for the EmployeeDepartment Junction table.

As a traditional Front End - Back End Access Relational Database Application, the forms are in one accdb and the tables in the other. The tables are linked, as can be seen in this image.

Linked Tables In Access FE, Identified by Arrow Icon

Employees and their assignments are managed through a traditional main form and subform design.

Main Form- Subform in Design View

Here's the same form in single form data entry view.

Main Form - Subform in Single View for Data Entry

After working with this Access Relational Database Application for a little while, the developer realized that it would be helpful to know when an employee leaves the organization. We want to add an Inactive Date to the Employee table.

However, not being experienced, the developer first tried to add the new field in the Front End, with this error.

Error Modifying Linked Table from Front End

A more experienced developer would immediately realize the problem and know how to solve it. Nonetheless, it does come up regularly, so I'm going to show you the next steps.

Open the Back End accdb with the tables. Open the Employee table in Design View.

Source Table in Back End accdb Open in Design View to Add Inactive Date

Now the appropriate field can be added and saved in the table.

InactiveDate Field Added in Source Table in Back End

Close the Back End accdb with the modified table in it. Reopen the Front End accdb. The new field should now be visible in the Linked Table. If it's not (and that's highly unlikely, of course), use the table relinker to refresh the link.

Refresh Links with Linked Table Manager

Now the new field can be included in the form's recordsource.

New InactiveDate Field Available for the Form's Recordsource

And a control for it can be added to the form.

New Field Available and Added to the Form.

Users Really DO Care About Your Interface

Last week I got a first-hand look at a user's reaction to a poorly designed interface. It made me smile.

I was in for a minor, routine medical procedure. The pre-op nurse was asking questions and entering my answers into the clinic's on-line medical records program. She noticed me staring at the screen and, perhaps thinking I was getting impatient (pardon the pun), felt she needed to apologize for how long it was taking. I assured her that I was mostly curious about the input process for professional reasons, having been a Relational Database Applicaton developer for many years. That prompted her to elaborate.

The system she was using required her to put in a couple of data points, then click a button or link to move to a different screen to add a few more data points, then move to a different screen for something else, and so on. She then explained that at her other job, they had a different system that allowed her to just scroll down as she entered data, without clicking and and waiting for a new screen to load. Much more efficient, at least in her opinion.

That told me a couple of things. First, she has joined the so-called Gig Economy and has, for better or worse, a variety of work environments to compare. Probably, she's not the only one doing that these days.

But more importantly, it told me that interface design is a big deal. I doubt that this particular nurse would give up a particular job because of it, but I would bet that if it came down to a choice between the two positions she holds, she is very much aware of, and concerned about, the usability of the tools available at the two.

Multi-Purpose Forms in an Access Relational Database Application

Microsoft Access is very flexible development tool. Seasoned developers know this, and take advantage of that flexibility in different ways. For example, different interface objects can be used for two or more purposes. A recent question at UtterAccess gave me an opportunity to create a small demo to illustrate how re-use of objects works.

In this demo Access Relational Database Application, I used one of the arguments for the DoCmd.OpenForm action to change the Data Mode for a single form.
Here's the full line of code, taken from two different procedures: 

    DoCmd.OpenForm FormName:="frmHousehold", View:=acNormal, DataMode:=acFormAdd

which runs from the Click Event of one Command Button. 


    DoCmd.OpenForm FormName:="frmHousehold", View:=acNormal, DataMode:=acFormEdit

which runs from the Click Event of the other Command Button. 

Two Command Buttons Open One Form in Two Different Data Modes

This is a very simple illustration of the re-use of objects. Instead of creating two different objects--add and edit forms in this case--to do two very similar tasks, we re-use the same object but vary the way it is used. Because adding records to a table, and editing records in that same table are almost identical tasks; it's obvious that we can simplify the Access Relational Database Application this way.

This is a simple example of a much broader set of design principles and practices. If you want to learn more, here's an excellent YouTube video by Phillip Steifel.
 
 

Pouring a Concrete Foundation After the Roof Goes On

Sometimes, when I'm cruising Access forums for questions I might be able to answer, I get the feeling that Access is simultaneously a really fantastic application-development tool and a really subtle trap for the unaware.

I'm talking about valid, proper, appropriate, NORMALIZED Relational Table Design, of course. Think, for example, of color options or size options for articles of clothing. Shirts come in Small, Medium, Large and Extra Large. Socks come in Blue, Black, Argyle or white. And so on. You've all seen "tables" in Excel worksheets, with one column for each option. In Excel they are the only option. You have to have different columns for each option.  And in the case of two options, you might even have combinations like "Small Blue", "Medium Blue", "Large Blue" and "Extra Large Blue", along with all of the other possible combinations. The inevitable result is multiple columns stretching out to the right as far as the eye can scroll.

Unfortunately, those columns make their way into Access tables as fields as well.

But in Access forums, the problem table design is seldom broached in the initial post, although I have seen a few direct questions about it.

The question often presents itself as a problem with designing an interface to handle multiple fields  Should you use a check box to select one of those options, or some other interface element that addresses each of those fields somehow?

Sometimes the question asks for help in constraining data when two or more fields are available, only one of which should be filled and the others left blank . If you pick a Large size shirt, you can't also have it be Small. Black socks can't also be White socks.

Sometimes it shows up as a problem getting results out of a query with that kind of inappropriate table design. How do I write a query to get the Shirt Size we sell most often?

There are many variations, but they all boil down to lack of Normalization, and I compare that to trying to go back and putting in a foundation under a house when you've already put up walls and a roof. It's possible, but it's not going to be easy and it all might collapse on you.

If you plan to create an Access Relational Database Application, and if you don't know what I am talking about here, I think you owe it to yourself to invest a few hours in your peace of mind by learning as much as you can about the Rules of Normalization BEFORE you try to put up walls and a a roof.  The nicest roof in the world won't keep the rain out if the building collapses because it lacks a foundation.

The reason this is a problem is that, as I said at the beginning, Access is really open and easy to get started. It's tempting to import a working spreadsheet and throw an interface on. And, unfortunately, that can be done without understanding what the consequences will be.

Be safe, stay dry. Normalize first.