数据库设计思想（Database design idea）.docVIP

数据库设计思想（Database design idea） Key: An entity cannot have neither primary key nor foreign key. An entity in a leaf can define a primary key, or a primary key (because it has no descendants), but must have a foreign key (because it has a father). The design of primary key and foreign key occupies an important position in the design of global database. Base table: The base table is different from the middle table and the temporary table because it has the following four characteristics: (1) atomicity. The fields in the base table are non - decomposed. (2) originality. The record in the base table is the record of the raw data (basic data). (3) deductive. You can derive all the output data from the data in the base table and the code table. (4) stability. The structure of the basic table is relatively stable, and the records in the table are to be kept for a long time. After you understand the nature of the base table, you can distinguish the base table from the middle table and the temporary table when you design the database. Paradigm: The first paradigm: 1NF is an atomic constraint on attributes, requiring attributes to be atomic and can not be decomposed; Second paradigm: 2NF is the only constraint on records, requiring records to have unique identifiers, namely, the uniqueness of the entity; Third paradigm: 3NF is a constraint on field redundancy, that is, no field can be derived from other fields. It requires no redundancy in the field. (paradigms can only serve as reference standards, not necessarily fit.) How about many to many relationships?: Nature is independent of their connections, stored in separate tables, but such entities do not exist because they extract ties Method for determining primary key PK: PK is an inter table connection tool for programmers. It can be a digital string without physical meaning, which is automatically added by 1. It can also be a combination of field names or field names with physical meaning. But the former is better than the l