When to use NULL and when to use an empty string?

I'm interested mainly in MySQL and PostgreSQL, but you could answer the following in general:

• Is there a logical scenario in which it would be useful to distinguish an empty string from NULL?

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What would be the physical storage implications for storing an empty string as...

• NULL?

• Empty String?

• Another field?

• Any other way?

Let's say that the record comes from a form to gather name and address information. Line 2 of the address will typically be blank if the user doesn't live in apartment. An empty string in this case is perfectly valid. I tend to prefer to use NULL to mean that the value is unknown or not given.

I don't believe the physical storage difference is worth worrying about in practice. As database administrators, we have much bigger fish to fry!

StackExchange Database Administrators Q#59, answer score: 90