Null values indicate that data is missing or unknown, and if you don't take steps to handle them, you could wind up with runtime errors or erroneous data.
These Access pointers will help you understand and effectively address null values in various situations. This article is also available as a PDF download. Where errors are concerned, null values are an equal-opportunity menace. If an unhandled null value doesn't generate a runtime error, it'll show up in erroneous data. Neither problem is your run of the mill "oops, there's a bug" error. In fact, an unhandled null value is the sign of a lazy or inexperienced developer.
When null values are acceptable values, and they often are, you must handle them upfront and aggressively. You can't handle a value properly if you don't understand its nature. A common misconception is that a null value is simply an empty field or no value at all. That's not true. A null value indicates that the data is missing or unknown. Occasionally, a null value does mean that the data doesn't exist or isn't valid for that particular record, but the concepts aren't interchangeable.
Since Access allows null values, it's your job to determine whether you want to store them. Generally, the data will be your best guide. If the nature of the data requires that all data be present to save the record, you can handle null values at the table level. Simply set the field's Required property to Yes and bypass the problem. Be prepared for the rules to change. Few applications are so tight that nulls aren't present.
If users need the flexibility to create records without entering all of the data at the time they create the record, you have a choice. Allow the table to store a null value or use a default expression that stores an appropriate text message, such as "NA" or "Pending. Unfortunately, this solution works only for text fields.
For numeric fields, you could use a default value of 0, but that might cause trouble in the long run because functions handle Null and 0 differently see 7. In addition, the Default property works only for new records. That means that you can't apply this solution to existing records. The truth is, it's usually easier to handle null values than it is to usurp them in this fashion.
Don't try to find null values by equating them to anything else. The following expressions return an error, regardless of anything 's value:. As far as Access is concerned, Null doesn't equal anything. This isn't always true outside Access. Once you decide that null values are acceptable, it's your job to accommodate them throughout the application.
For instance, to find null values in a query, you'd enter Is Null in the appropriate field's Criteria cell. For instance, the use of IsNull in the following If statement handles a potential runtime error when null values exist:. Access won't always work with null values as you might expect. If you allow them, be prepared for surprises. For instance, a simple expression such as. Instead of returning just the Subtotal, as you might expect, the expression returns Null.
That's because any equation that encounters a null value will always return Null. Although it's a nuisance, it makes sense. You can't evaluate an unknown value. If your data contains null values, use the Nz function to protect your expressions from this error. Specifically, Nz returns a value other than Null when it encounters Null as follows:. In this case, Nz returns 0 when Shipping equals Null.
Use Nz in criteria and VBA expressions. Access projects don't support Nz. In 3, you learned that Null doesn't equal anything. That's true, as long as you're using native functions and VBA. So, since the students without grades are left out when the Criteria is set to Is Not Null , that leads me to believe that a student without a grade is null. So, I am very confused why setting the Criteria to Is Null , just leaves me with no results. I am very new to Access, so if there is some important information I need to provide for adequate help, please let me know!
Very likely your join is a standard join. If the child record that contains the grade does not exist, then they don't appear in your query. Fire up the query builder, and double click on the join line - change it to a left join the option that says to include all students. Now, when you run the query, records with a grade row will show, and those without will result in the grade column being null.
Now, both of your conditions can and should work. It not the condition that is failing but your query not showing any students without a grade regardless of your condition that is the problem.
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Active 2 years, 8 months ago. Viewed times. I am making a grade entering system in Access, and I am running into some issues with a query. So, since the students without grades are left out when the Criteria is set to Is Not Null , that leads me to believe that a student without a grade is null So, I am very confused why setting the Criteria to Is Null , just leaves me with no results.
Improve this question. Looks strange. What is Grade field data type? Do you apply criteria in the query directly to the table to another query? The query may change data type — Sergey S. This is a text field with letter grades? Then the field is not null. An empty string looks like null but they are not the same.
This is why I do not allow empty string in text fields. Somehow field has empty string. Is this the same as doing Is Null , and if so do you know why Is Null is not working? June7 It is a text field. But if they are not null, why do the empty strings not appear when the criteria is set to Is Not Null? An empty zero-length string is different from Null. You should test for both cases e. Show 2 more comments.
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