A detailed explanation of the foreign key key value in MySQL

If key is empty, then the column value can be repeated, 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

If the definition of a column satisfies many of the 4 cases mentioned above, for example, a column is both a PRI and uni
Then "desc table name", the displayed key value is displayed according to the priority level Pri->uni->mul
So at this point, the PRI is displayed

A unique index column can be displayed as a PRI, and the column cannot contain null values, and the table does not have a primary key

A unique index column can be displayed as Mul if multiple columns make up a unique composite index
Because although the multiple-column combination of indexes is unique, such as Id+name is unique, no single column can have duplicate values
As long as Id+name is the only one.

A detailed explanation of the foreign key key value in MySQL