What Is Unique Constraint?

Ever had two people with the same last name? A unique constraint can prevent that at the database level, helping you keep track of your data. A unique constraint is a column restriction within a table, which dictates that all values in that column must be special though they may be null. Although the table can have multiple entries with the same value in a special column, you cannot have multiple rows with identical values in any other row. Unique constraints can be specified on an existing column when you want to add a new table or view to your database or add columns to a current table or view. It can help enforce data integrity within your database by ensuring that no duplicate records exist. When a column is UNIQUE, it cannot contain any null values. Ensure a column is UNIQUE and cannot have null values. Must specify the column as NOT NULL; interestingly, these are the primary key’s two main attributes. Defining both characteristics in a newly-created column should be seriously considered for the primary critical designation. When a constraint is determined, the system automatically assigns a unique number to each new row in the table. This number is called a UUID or a globally unique identifier. The null value is a constraint that prevents you from creating duplicates. This constraint, however, does not tell you whether or not a value stored in any given field is unique-it prohibits duplicate values. Therefore, it is impossible to say that the value in that null field is not unique, as nothing is stored there. It cannot compare a null value to an actual value. For example, one Cannot compare the Queen of America cannot reach Queen of America to the Queen of England because the Queen of America is a null that does not exist.