SQL UNIQUE Constraint indicates that a column or set of columns should contain unique values, meaning that no two values of "Ename" column (if Ename has a UNIQUE constraint) should be same.
Example 1 of SQL UNIQUE Constraint
CREATE TABLE Sells ( bar CHAR(20), beer VARCHAR(20), price REAL, UNIQUE(bar,beer) );
Example 2 of SQL UNIQUE Constraint
CREATE TABLE Sells ( bar CHAR(20) UNIQUE, beer VARCHAR(20) UNIQUE, price REAL );
Both the above example are different.
Difference between Unique & Primary key.