SQL Unique Constraint
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The UNIQUE constraint ensures that all values in a column are distinct.
For example, in the following CREATE TABLE statement,
CREATE TABLE Customer
(SID integer UNIQUE,
Last_Name varchar (30),
column "SID" has a UNIQUE constraint, and hence cannot include duplicate values. Such constraint does not hold for columns "Last_Name" and "First_Name". So, if the table already contains the following rows:
Executing the following SQL statement,
INSERT INTO Customer VALUES (3, 'Lee', 'Grace');
will result in an error because '3' already exists in the SID column, thus trying to insert another row with that value violates the UNIQUE constraint.
Please note that a column that is specified as a primary key must also be unique. At the same time, a column that's unique may or may not be a primary key. In addition, multiple UNIQUE constraints can be defined on a table.