UNIQUE constraint uniquely identifies each row in a table.
You can have multiple UNIQUE constraints in different columns of a table.
There are several different forms of syntax as the database system used
SQL UNIQUE Example for MySQL database
The above statement creates the table 'people' with 4 columns where the column 'id' has a different value for each row of the table.
If we try to insert a row with an 'id' that already exists, we will fail, and we will not insert it.
Example SQL UNIQUE for Oracle databases, SQLSERVER, ACCESS