Can MySQL\'s UNIQUE Constraint Handle Empty Values in a Column?

Unique Constraints with Empty Values in MySQL

Consider a scenario where you have a field storing product codes that must be unique, but certain products lack designated provider codes. The question arises, is it possible to define a unique constraint that accommodates this scenario in MySQL?

Answer:

Yes, it is feasible to create a unique constraint that allows empty values in MySQL. As mentioned in the MySQL reference manual (version 5.5):

"A UNIQUE index creates a constraint such that all values in the index must be distinct. An error occurs if you try to add a new row with a key value that matches an existing row. For all engines, a UNIQUE index allows multiple NULL values for columns that can contain NULL."

Therefore, for a column that permits NULL values, despite its NOT NULL property, it is perfectly valid to define a unique constraint without leading to conflicts. This allows for situations where certain products do not possess assigned codes, enabling them to be inserted into the database without violating the uniqueness requirement.

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