Yes, another NULL vs empty string question.
I agree with the idea that NULL means not set, while empty string means ‘a value that is empty’. Here’s my problem: If the default value for a column is NULL, how do I allow the user to enter that NULL.
Let’s say a new user is created on a system. There is a first and last name field; last name is required while first name is not. When creating the user, the person will see 2 text inputs, one for first and one for last. The person chooses to only enter the last name. The first name is technically not set. During the insert I check the length of each field, setting all fields that are empty to NULL.
When looking at the database, I see that the first name is not set. The question that immediately comes to mind is that maybe they never saw the first name field (ie, because of an error). But this is not the case; they left if blank.
So, my question is, how do you decide when a field should be set to NULL or an empty string when receiving user input? How do you know that the user wants the field to be not set without detecting focus or if they deleted a value…or…or…?
Related Question: Should I use NULL or an empty string to represent no data in table column?
I almost never use NULL when referring to actual data. When used for foreign keys, I would say that NULL is valid, but it is almost never valid for user entered data. The one exception that would probably come up quite regularly is for dates that don’t exist, such as an employee database with a ‘termination_date’ field. In that case, all current employees should have a value of NULL in that field. As for getting them to actually enter a null value, for the values that truly require a null value, I would put a checkbox next to the input field so that the user can check it on and off to see the corresponding value to null (or in a more user friendly manner, none). When enabling the checkbox to set the field to null, the corresponding text box should be disabled, and if a null value is already associated, it should start out as disabled, and only become enabled once the user unchecks the null checkbox.