MySQL NOTE 1

: https://dev.mysql.com/downloads/mysql/(Community edition open source free)

Data redundancy: Two tables contain the same field, in layman's words is redundant data;

Data loss integrity: data loss;

Primary key: Uniqueness, (null primary key, i.e. loss of entity integrity)

FOREIGN key: The data in this table originates from another table (cannot be arbitrarily changed)

Data that does not exist in the primary key table in the foreign key is referred to as "incomplete reference"

The type, scope, and format are not correct, known as Lost domain integrity;

MySQL NOTE 1