Meaning of key in MySQL database

Four types of Key:primary key, Unique key, key, and Foreign key.

1. If key is empty, then the column value can be duplicated, indicating that the column has no index, or is a non-leading column of a non-unique composite index
2. If key is a PRI, then the column is part of the primary key
3. If key is uni, then the column is the first column (the leading column) of a unique value index and must not contain a null value (NULL)
4. If key is Mul, the value of the column can be repeated, which is a leading column of a non-unique index (the first column) or a component of a unique index but can contain null values

Meaning of key in MySQL database