📅  最后修改于: 2022-03-11 14:56:17.553000             🧑  作者: Mango
The entity integrity constraint states that primary key value can't be null.
This is because the primary key value is used to identify individual rows in relation and if the primary key has a null value, then we can't identify those rows.
A table can contain a null value other than the primary key field.