Of null VARCHAR fields and empty strings

How to represent a blank user input is a reasonably innocent design question. On a couple occasions though I have been bitten by systems that have been designed without considering that question seriously. Maybe it’s a clear choice to some, but today’s post makes an argument that in representing an empty input as a null is better than using a empty or blank string.

Today’s post is about how represent nothing.

A black box

The box represents the emptiness of input forms implemented in PHP.

Although there are lots of cases where an application may take input, handling (human) user input is one of the most interesting because it’s often highly varied. Recently I worked with a organization that has volunteers enter the manufacturer name of the computer they use for an activity. For a computer made by Dell Inc. the data entered by those volunteers ranged from “Dell Inc.” to “Dell Inc” to “Dell” to “dell” and finally, also “”. Each user’s input was then stored in a database along with other user inputs. Since the user’s inputs were free form text VARCHAR columns were used to store them. We’ll ignore the data normalization issues involved in this kind of data for today and instead just focus on what happens when the input is “”.

The first issue with storing unused inputs as “” is that it makes determination of intent more difficult. Take the two following side-by-side tables:

MyColumn
“”
NULL
MyColumn
“”
“”

In the first table it is possible to tell the difference between one row which has no data for ‘MyColumn’ and the other row which has no data for the column. In a number of cases there may be no significant difference to the end user between empty and non-existent, but the point is to not have to make that distinction until the last possible moment.

The other thing I noticed was using empty field values in fields used as join columns. Going back to the organization I worked with they did not have a separate manufacturer identifier and used the VARCHAR manufacturer column to join against another table. The biggest danger with this approach is that if blank rows are not explicitly excluded from the join then a significant number of incorrect records may be returned.

The linked GitHub gist has T-SQL (for SQL Server) to create a database and an example query that can be used to experiment with blanks in join conditions. With appropriate indices there is not a huge difference between

from table t 
inner join t2 
on t.id = t2.foreign_key

and

from table t 
inner join t2 
on t.some_varchar = t2.some_varchar

or

from table t 
inner join t2 
on t.some_varchar = t2.some_varchar
where t.some_varchar <> ''

The first two of three SQL fragments above look better to my eye by not having to accommodate the overloaded meaning of ” which can be accomplished by storing the absence of user input as NULL and letting a blank just be a blank.

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