What is a relational database?
Consists of the association between the data table and the data table
Characteristics of relational databases
Each data with the same attributes is stored in a table independently, and the user can add, delete and modify the data in the table without affecting other data in the table
Basic term
Key: one or more columns used to identify the row
Primary Key (Primary Key): referred to as the primary key, a uniquely identified candidate key, there is only one primary key in a table
Candidate Key: It is an attribute set that uniquely identifies a row in the table and does not contain redundant attributes
Common Key: Expressed by compatible or identical attributes or attribute groups. If it has, it is called a common keyword
Foreign Key: Also known as foreign key. If a common key is the primary key in a relationship, then this common key is called the foreign key of another relationship
What are the integrity rules?
Entity integrity rule: the primary attribute cannot be a null value
Referential integrity rules: If the foreign key R1 in the relationship matches the primary key in the relationship R2, then each value of the foreign key must be found in the value of the primary key in the relationship R2 or be a null value
User-defined integrity rules: for a specific column of constraints in the actual database
(Integrity constraints that entities and references must meet)