What are the characteristics of a unique index in MySQL?

Here are some key features of MySQL unique indexes:

1. Uniqueness: A unique index ensures that the values in the column are unique, meaning that each index key value can only correspond to one data record.
2. Quick search: Unique indexes can speed up data retrieval and improve query efficiency. When executing a query, MySQL can directly locate the data record corresponding to the index key value without the need for a full table scan.
3. Avoiding duplicate data: unique indexes can enforce that data in the database is not duplicated, ensuring data consistency and integrity.
4. Null values are allowed in a unique index. If a column has a unique index, it can have multiple null values but the column itself is still unique.
5. Crossing columns: A unique index can include multiple columns, also known as a composite unique index. If a composite unique index is set, then the combination of index key values must be unique.

It is important to note that a primary key is different from a unique index. A primary key is a special type of unique index that requires the indexed key value to be not empty and unique throughout the entire table.