Designing a database has several rules for proper design, which one of the following is not a rule?

Responses

One column in the table should be unique to identify the row of data $($primary key$).$

One column in the table should be unique to identify the row of data $($primary key$).$

Only numerical data should be in the database.

Only numerical data should be in the database.

Each column should have same type of data and not mix data types.

Each column should have same type of data and not mix data types.

Each column must have single values.