1. Introduction to related terms of relationship
Attribute: Several characteristics that describe things are called attributes. Such as student ID, name, position, age, etc.
Domain: A collection of value ranges for attributes. For example, the gender is male, female, and the length of the student ID is 8 digits. Generally, in the relational data model, a restriction is added to the domain, and all domains are atomic data (Atomic Data).
For example, integers and strings are atomic data, and collections and arrays are non-atomic data. This restriction of the relational model is called the first normal form (1NF) condition.
Degree or degree (Degree): R represents the name of the relationship, and n is the degree or degree of the relationship.
Candidate Key: If the value of a certain attribute or attribute group in the relationship can uniquely identify a tuple, it is called the attribute or attribute group as a candidate code.
Primary Key: Primary key. If there are multiple candidate codes for a relationship, select one as the primary key.
Prime Attribute: The attribute that contains any candidate code is called the primary attribute, and the attribute that does not contain any candidate code is called the non-primary attribute.
Foreign Key: The attribute or attribute group in the relational pattern R is not the code of the relationship, but it is the code of other relations, then the attribute set is a foreign key for the relational pattern R, also called a foreign key.
All-key (ALL-key): All attribute groups of the relational model are candidate codes for this relational model, which are called full keys.
2. The type of relationship
Basic relationship: The actual table is the logical representation of the actual stored data.
Query table: the table corresponding to the query result.
View table: A table derived from a basic table or other view tables is not stored independently in the database. The database only stores definitions, so it is also called a virtual table.
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