MySQL DatabaseLesson 54Free preview

MySQL Unique

This lesson is part of MySQL · All levels

Summary

The UNIQUE constraint ensures that each value in the column is different, so that the double data cannot be stored. Unlike the primary key, a table may have multiple UNIQUE columns and on MySQL the value NULL remains allowed (and is considered to be non-equal). UNIQUE is often used for email, identity number, or code that must be unique.

Key points

UNIQUE prevents duplicate values on columns or column combinations
One table can have many unique constraints
MySQL allows more than one NULL in the UNIQUE column
It can be defined in one column or multiple columns at once.
Name the constraint so it's easy to reference when it's deleted.

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