A lot of material has been created in support of using functions in Access queries. They are a key feature in making Access queries the powerhouse of data processing that Access is famous for. Just making an AI inquiry about “Using Functions in Access Queries” brings up a wealth of blog articles and YouTube videos on the subject. However, rather than regurgitate the same information that you can get from any Microsoft Knowledge Base article on the subject, I prefer to expand on the material already available and add additional lessons learned and nifty tips on handling issues that can only be experienced by actually using functions in Access queries first-hand.
There are two major sections in the Access query design grid where you can use functions: The “Field” row, and the “Criteria” row. Functions placed in these sections are handled differently depending on what section you put them in. I’ll explain how to use both built-in and custom functions in both sections, but first there are a few things you need to keep in mind when using functions in general.
General Considerations:
Columns in Access queries are specific as to the data type for the data that the column contains. Regardless of whether you use a built-in function or a custom one, the returned value of the function should be compatible with the column’s data type, or you will see some strange results and even get errors when you try to run the query. If your function contains input parameters, those parameters must be of the same data type as specified in the function’s definition. Again, failure to use the correct data type will produce strange results and even errors when you try to run the query.
There is no difference in syntax when using either built-in or custom functions in either the Field line or Criteria line of the Query Design Grid. The main difference lies in how the function is executed. Functions without parameters that are used in the Field line are executed only once. Functions with parameters that are used in the Field line are executed for every record in the data source and every field that the function participates in. For example, if the function is called in two fields of the query and the query data source has 1,000 records, then the function will be executed 2,000 times. It is easy to see how using functions here can impact query performance.
If the function is used in the Criteria section, it is executed only once for each time that the function appears in the criteria. Using functions in Criteria has a much less potential for causing performance issues. If you have functions in both sections of your query and are experiencing slowness, try breaking the query into two queries where the first query has only criteria, and the second query has only functions in the Field row. Use the results of the filtered query as the data source for the query with the functions in the field row. This will result in a reduced number of rows that have to be processed.
Tip: If your function does not have parameters, be sure to include the empty parentheses at the end of the function name to indicate to Access that you are referencing a VBA function.
Built-in Functions:
By definition, built-in functions are functions already constructed by Microsoft for use in Access and have very specific design specifications for using them in queries. If you are unfamiliar with Access’s Built-in queries, then I recommend you take a moment to do a Google search and review them so you will know what you have available for use when the need arises. Be particularly attentive to the number and type of input parameters required, and the type of data returned.
Tip: You can use the Expression Builder to get built-in functions displayed in the builder’s IntelliSense.
Troubleshooting Built-In Functions:
If a built-in function is being used in the field row in the query and it is causing an error when you try to run the query, it can be difficult to troubleshoot what is causing it, especially if the error is due to bad data being fed to the function as a parameter. Built-in functions have very little to no error handling capability.
It is much easier to troubleshoot these issues if you put the function in a custom VBA function and call the VBA function in the query, being sure to provide any parameters called for by the function in the VBA code. Put local error handling in the VBA function so if any part of the function that you are testing fails, you get an error message. You can also test the function’s supplied parameters to see what values are being transferred to the function. Also, be sure to include the record identifier value as a VBA function parameter so you can identify what record has the values that are causing the problem.
Custom Functions:
You can also include custom functions that you build in basic VBA modules in your queries. Custom VBA functions in your queries can really give them processing superpowers.
For example, in a payroll app, you may have a need to take the shifts that employees work and divide them into periods (“Morning”, “Evening”, and Night”) which pay at different rates, and which start and end at specific times. Putting the logic for calculating how many hours of each employee’s shift fall into which period can be handled by a custom VBA function in a query that has the employee shift date, start, and end times as parameters.
Tip: You can use the Expression Builder to get your custom functions displayed in the builder’s IntelliSense.
In Summary:
| Query Section | Function Type | Execution Frequency | Performance Impact |
| Field Row | No Parameters | Once | Low |
| Field Row | With Parameters | Per Record x Field | High |
| Criteria Row | Any | Once per appearance | High |
From experience, combining built-in and custom functions allows you to build streamlined, cascading queries that reduce complexity and improve performance. With the right approach, you can eliminate temporary tables and transform multi-step processes into elegant, efficient solutions.
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