MySQL indexing problem

Note: The index is generally suitable for frequently queried data, can improve the efficiency of the query, but not suitable for frequent use of the increase, deletion, modification of the data, the impact of inefficient.

1.unique key-> (unique index) there can be multiple in a table that acts as a constraint: avoid duplicate values.

2.primary key-> (primary key index) there can be only one in a table. is usually the self-increment ID.

3.key Normal index.

Second, the syntax of the index:

MySQL indexing problem