Why use Unique when Primary key on same key is already defined

Question

While reading Database Systems, I have encountered through a query as shown below:

 CREATE TABLE works ( eid INTEGER NOT NULL, did INTEGER NOT NULL, pcttime INTEGER, PRIMARY KEY ( eid, did ), UNIQUE ( eid ), FOREIGN KEY ( did ) REFERENCES dept );

Since primary key on eid,did is already defined, why would we require unique on eid? Because each eid,did pair is already unique.

Chris Mack · Accepted Answer · 2017-10-17 23:18:37Z

4

The primary key is on two columns, so you could still have non-unique eids. E.g.,

eid did 1 1 1 2

This doesn't violate your primary key, but would violate your unique constraint.

answered Oct 17, 2017 at 23:18

Chris Mack

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1 Comment

Dipendra543 Over a year ago

Thanks for the explanation!!

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