SQL DatabaseLesson 54Free preview

SQL Unique

This lesson is part of SQL · All levels

Summary

The UNIQUE constraint ensures that all values in the column are different from each other, but still may contain NULL. Suitable for user email or double-digit identification numbers. This constraint can be applied per column or as a combination to several columns.

Key points

UNIQUE guarantees no two rows with the same value on the column.
The value of NULL remains permissible and in MySQL can be more than one NULL.
It can be written in the UNIQUE column or through the ADD CONSTRAINT... UNIQUE column.
A unique constraint automatically creates an index to speed up the search.
The primary key is stricter than the unique because it does not allow.

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