Difference between Unique Key and Primary Keys

Question

I came across the following SQL in a book:

CREATE TABLE \'categories\'(
id SMALLINT NOT NULL AUTO INCREMENT,
category VARCHAR(30) NOT NULL,
PRIMARY KEY(\'id\         


        

Answer 1

Similarity

Both a PRIMARY and UNIQUE index create a constraint that requires all values to be distinct (1).

Difference

The PRIMARY key (implicitly) defines all key columns as NOT NULL; additionally, a table can only have one primary key.

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(1) Each NULL value is considered to be distinct.