Keys in database management system

Welcome to my “Keys in database management system.”

Key

Super key: a set of attributes that uniquely identifies a tuple in a relationship Super key called a relational schema

Candidate key: A super key that does not contain redundant attributes is called a candidate key.https://www.blogger.com

Primary key: The user selects the primary key of a candidate key for the tuple identification

Foreign key A foreign key of the relational schema R1 if the attribute set in relational schema R1 is not the primary key of R1 but the primary key of another relation R2.

Combined with the following examples for a specific explanation:

Assuming the following two tables:

Student (Student ID, Name, Sex, ID Number, Teacher ID)

Teacher (teacher number, name, salary)

Super key:

By the definition of super-key shows that the student table contains a student ID number or any combination of this table is super-key. Such as (student number), (student number, name), (ID number, gender) and so on.

Candidate key:

The candidate key belongs to the super key; it is the smallest super key, which means that if you remove any of the candidate key attributes it is no longer a super key. Student table candidate key: (student number), (ID number).

Primary key:

The primary key is the one inside the candidate key, which is man-made. For example, in the student table, we usually let the student ID be the primary key. In the teacher table, let the teacher number be the primary key.

Foreign key:

The foreign key is relatively simple; the foreign key in the student table is "teacher number." The foreign key is mainly used to describe the relationship between the two tables.

Paradigm:

At the heart of the relational model, a principle is a notion of "normalization," which is the process of organizing a database into a database that minimizes redundancy while preserving the integrity of stored data. Normalized databases are databases that conform to the rules of the relational model, and these rules are called paradigms.

The first paradigm (1NF):

1NF:

Each column of a database table is an indivisible basic data item. You cannot have multiple values in the same column, that is, an attribute in an entity cannot have multiple values or cannot have duplicate attributes.

Guiding Principles:

1. Each property of an array can contain only one value

2. Each array in the relationship must contain the same number of values

3. Each array in a relationship must not be the same

2NF:

If a data table already satisfies the first paradigm, and the value of any non-primary key field in the data table depends on the primary key field of the data table. (Does not contain partial dependence)

Third Normal Form (3NF):

3NF:

If a data table already satisfies the second normal form and there is no functional trust between the data values of any two non-primary key fields in the data table, the data table satisfies the third normal form. (Does not contain non-primary attributes of the key to rely on the delivery)

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