Why Mysql cannot create a foreign key

When you create a foreign key in Mysql, you will often encounter problems and failures. This is because Mysql still has many details to pay attention to. I have summarized and reviewed the relevant information and listed the following common causes.

1. the type or size of the two fields are not strictly matched. For example, if one is int (10), the foreign key must also be set to int (10), not int (11), or tinyint. In addition, you must determine whether one of the two fields is signed, and the other is unsigned (I .e., unsigned). These two fields must be exactly consistent,