Defining Unique Keys
The definition of unique keys is syntactically very similar to primary keys.
ALTER TABLE
ADD UNIQUE ( {, }... )
Likewise, unique keys can be defined as part of the CREATE TABLE SQL statement.
CREATE TABLE TABLE_NAME ( id_col INT, col2 CHARACTER VARYING(20), key_col SMALLINT, ... CONSTRAINT key_unique UNIQUE(key_col), ... ) CREATE TABLE TABLE_NAME ( id_col INT PRIMARY KEY, col2 CHARACTER VARYING(20), ... key_col SMALLINT UNIQUE, ... )Read more about this topic: Unique Key
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