Is uniqueness of values required for fields included in a database table index?

The uniqueness of values for fields included in a database table index is not always mandatory. It depends on the type of index:

• Unique index (UNIQUE) guarantees that the values in the indexed fields will be unique.
• Regular index (non-unique) does not require uniqueness and can contain duplicate values.

Therefore, if the index is created as unique, the values must be unique; otherwise, they do not have to be.