is the unique KEY in MySQL a constraint or an index?

Answer from: https://zhidao.baidu.com/question/1863373387452612907.html

1. Relationship between the two

   The unique index contains a unique constraint, because the UNIQUE constraint is implemented through a unique index.

   To implement a unique constraint, the database enforces a unique index on the database

2. The two are the same.

   The operation that inserts a repeating column value into the table will fail.

3. The difference between the two

   The difference lies in the creation and deletion of the. indexes are created and deleted using Create/drop Index and constraints are established using ALTER TABLE TB ADD constraint, using drop constraint to delete

4. Welcome to the Vomit Groove

is the unique KEY in MySQL a constraint or an index?