1.count：
count(*) is not ignored, and one line is added. count(fieldname) only counts the number of rows where the field is not null. For non-nullable fields, this method is not as efficient as count(*).
2. The difference between table-level integrity constraints and column-level integrity constraints
Column-level constraints: when you add conditions, others are not allowed to operate the columns you add conditions
Table-level constraints: when you add conditions, others are not allowed to operate the tables you add conditions
3. Situations that may destroy referential integrity and its breach of contract