MySQL foreign key Add error1215

In the process of MySQL creating a table foreign key, due to improper operation, will prompt cannot add foreign KEY constraint error.

This error may be caused by the following reasons:

1. The data type does not match, and the key associated with the foreign key must be of the same data type and length.

2, the table type must be InnoDB

3. The character encoding of the table must be the same

4. The key associated with the foreign key may not exist

5. When the foreign key is set, the on delete or on Update event is set to NULL when the table definition must be selected allow null

MySQL foreign key Add error1215