this post was submitted on 29 Jun 2023
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Ehhh, I don't quite agree with this. I've done the same thing where I used a timestamp field to replace a boolean. However, they are technically not the same thing. In databases, boolean fields can be nullable so you actually have 3-valued boolean logic:
true,false, andnull. You can technically only replace a non-nullable field to a timestamp column because you are treatingnullin timestamp asfalse.Two examples:
A table of generated documents for employees to sign. There's a field where they need to agree to something, but it's optional. You want to differentiate between employees who agreed, employees who disagreed, and employees who have yet to agree. You can't change the column from
is_agreedtoagreed_at.Adding a boolean column to an existing table. These columns need to either default to an value (which is fair) or be nullable.
Using a nullable Boolean to represent 3 distinct states just adds confusion and complexity to your system. In most cases I would prefer to use an enum with 3 fields which is non nullable.
Yeah, this feels like "premature optimization". When you design your applications and databases, it should reflect your understanding of the problem and how you solved it as best as possible. Using
DATETIMEOFFSET NULLwhen you actually meanBIT NOT NULLisn't saying what you mean. If you already understand that you have a boolean option and you think you might need a timestamp to track it, use 2 columns. Or an audit table. So sayeth the holy SRP.To add to this: the DBMS may treat null as unknown rather than not set. This may be not immediately obvious or noticeable, but it means a check requires different syntax and even double checks for set and value. Using null as 'cleared' goes against the DBMS definition of what null means.
Sql server docs
As a side note: this difference in null behavior can become especially problematic when you use entity frame for to map tables and sql queries to C#. Because it's not obvious and may not be known or seen.