Entity types. Database design with ERwin Any entity different from other entities

Living in a three-dimensional world, we rarely notice the work of beings from other dimensions. And their presence is not denied even by religion. Do you know what astral entities are? How and why do they come to a person? Why are they harmful and how to protect yourself from them? Are you saying this is a hoax? Such naive confidence will quickly disappear as soon as you interest astral entities in something. Their types are so diverse that it is rather difficult to understand the results of their activities. Let's figure out what they are and why people are afraid of them.

General concept

There are several theories regarding these objects unknown to the common man. Drunvalo Melchizedek wrote that they are inhabitants who by chance came to our world. Astral entities live by their own rules. They are ignorant of human laws. However, the people themselves are interesting. The fact is that we differ from other inhabitants of the vast Universe in that we are able to generate energy. Our guests eat it. Everything is very simple. They themselves are not able to get food from space. But any person is very good at it by definition. It is arranged in such a way that its body and soul function in two streams, on which, like a bead on a thread, it “dangles” in space. A person continuously receives and processes the energy of the Universe and the Earth. We feel it as feelings, thoughts, emotions. Astral entities stick to the aura and take away part of this incredible wealth. But clean energy is not suitable for them. These creatures feed on low-frequency energy. In our understanding - evil, hatred, resentment, doubts and so on.

What do entities do with a person?

You are probably familiar with the term "possessed". It is applied to a person who, in certain circumstances, is characterized by inadequate behavior. The priests say that he is possessed by demons. These astral entities (photos make you horrified) have settled in the aura of the unfortunate. His will is suppressed partially or completely. They are led by creatures from another world. They provoke people to do strange things. They need the unfortunate person to experience negative emotions and make others do it. The energy emitted by a human being is not suitable for demons. They are truly afraid of the bright side of the personality. Therefore, they try to push a person to sin. Possession is an extreme case.

Subtle human bodies and entities

To understand the topic, we present a diagram. Imagine that a person is a balloon filled with air. Schematically, this is how the aura is often depicted. This ball has two inputs and outputs through which energy constantly flows in and out. Its average quantity is such that the volume remains unchanged. The content of the ball is exactly what the entities are after. But they will not be able to stick to a tightly stretched shell. This happens when the person is happy and satisfied. If a person often has negative thoughts, he is angry, offended, grumbling, indignant, envious, suffering (then list it yourself, based on experience), then the elasticity of the shell decreases. Or, in another way, dark spots appear in the aura. It is very easy for entities to get to these places and gain a foothold on them.

It should be understood that there are quite a lot of these around us. Everyone hunts for their own type of negative energy. If, for example, you are prone to jealousy, a larva will stick, which will provoke precisely this feeling. She will also call a friend who "feeds" on wine. Together they will push for a glass. Do not resist - the larva of alcoholism will also appear in the aura. And they will arrange a mountain feast on your energy, taking away the forces that are given to organize a happy life. The person himself will transform them into negativity in order to feed his unwanted neighbors.

Astral entities: types

We have listed the most common astral entities. Their classification, according to esoteric theory, is much broader. But even on the examples given, it will be possible to get a general idea of the methods of their work and the level of harm caused to a person. Let's take a closer look at each.

Lyarvy

Incubus and succubus

Photos of these objects sometimes appear in various sources. It is difficult to judge the authenticity of these images. In any case, experts call most of them fakes. In principle, the essence is not in the photographs. Demons are the source of many misfortunes. They drag a large number of people into the funnel of inferno (negative energy), forcing them to kill and die. But the demon cannot move into the first comer. He needs a suitable energy space for life. And it is created by the person himself with negative, destructive, dangerous thoughts and actions. They say about such people: they lost their conscience. But clean energy is most afraid of astral entities. They are afraid of divine, which means love that does not depend on anything. To people in whose aura it is present, they do not settle down.

Elementors

At the moment of death, the soul is released from the human body and goes to the astral plane. But there are exceptions. Sometimes, due to attachment, due to magical influence, or for other reasons, the soul does not want (or does not have the opportunity) to fly into the space allocated by the Lord for its existence. She settles into the aura of a loved one. Elementer cannot be called a negative entity in the literal sense. It exists due to the energy of the living, slightly weakening its field. However, it does not provoke destructive activity. He is not afraid of pure energies. In addition, due to the connection with the Universe, the elementary is able to protect the person who accepted it from worldly dangers. However, this condition is not considered natural. The essence is not able to go to the subtle worlds, it cannot leave the carrier's aura on its own. She loses the chance for a new incarnation, which is very bad for her personal fate and for the whole family.

Astral entities: classification according to L. G. Puchko

The deceitful spirit forces its victim to lie. A person falls into depression, loses touch with reality. As a rule, a deceitful spirit settles in those who suffer from any addiction (gaming, alcohol, drugs). This unfortunate one constantly, aimlessly, senselessly lies.
Lucifer enters the victim's aura on a full moon. It pushes a person to unreasonable tough aggression. A person cannot restrain impulses. He argues with everyone, scandals, is capable of violence, including sexual.
Archimania is an entity that chooses stingy people. The victim seeks to have as much as possible of what he considers valuable.
UFO appears in people obsessed with contact with aliens.
A nerve blocker makes a person suffer from pain.
A leech is an alien energy structure that attaches itself to people with a low level of vibration. The victim quickly gets tired, becomes irritable, often gets sick.
The shell forces the person to abandon naturalness. A person puts on a mask, becomes fake, like a bad actor.
A witch is an entity created by a sorcerer. It is meaningful and aimed at making the victim behave in a certain way. A kind of negative energy-informational program, popularly referred to as spoilage.

It should be noted that L. G. Puchko focused on harm to human health, highlighting the types of astral entities. They also have a negative effect on memory. The fact is that these formations take energy from the victim, forcing him to experience hunger, which is extremely harmful to his body and brain.

In addition, it is recommended to change the diet. It is not in vain that there are fasts in any religion. Restrictions in the sphere of physical pleasures contribute to the purification of energy. Naturally, first of all, it is necessary to abandon all bad habits.

It should be noted that not all entities can be dealt with on their own. Sometimes you should trust a specialist. Thus, the astral essence of a person (elementary) will not leave the aura as a result of prayers and diet. It should be removed by a special ritual. To combat the most common entities, they reprimand with prayers, sometimes in church. The possessed are treated in monasteries. Weak entities can be dealt with on their own. It is necessary to clear thoughts, get rid of negative emotions. That is, fill your entire field with light energy. There are special methods of multidimensional medicine that help to quickly deal with the problem. They consist in the fact that the patient is invited to read vibration series.

Prevention

Where there is attention, there is power! Strive to guide her towards love. This does not mean a feeling that brings a man and a woman together for procreation. Love is divine. This is a state of perfect happiness, when everyone is happy, nothing angers or irritates. The soul of every person yearns for it. Although on earth this is impossible. Only an angel manages to stay in a state similar to the ideal. But this does not mean that a person does not have the right to strive for divine love. Even the intention will already fence off most of the entities from you. They will not be able to get anything, therefore, they will lose interest in your aura. In fact, the world is very harmonious. Astral entities are drawn to those who themselves create satisfactory conditions for them. Why waste so much energy? What do you think?

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twisted pair

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optical fiber

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Megabyte

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Consequences

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withdrawal mechanism

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States

small business

big business

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2.3. Entity classification

Three classes of entities

It's time to clear up the terminology. K.Date [ 3 ] defines three main entity classes: rod, associative And characteristic, as well as a subclass of associative entities - designations.

Rod entity (pivot)

Core entity (kernel) is an independent entity (it will be defined in some detail below).

In the examples considered earlier, the rods are "Student", "Apartment", "Men", "Doctor", "Marriage" (from example 2.2) and others whose names are placed in rectangles.

Associative entity (association)

Associative entity (association) is a many-to-many ("1-to-many", etc.) relationship between two or more entities or entity instances (as in example 2.4). Associations are treated as full entities:

they can participate in other associations and designations in the same way as core entities;

may have properties, i.e. have not only a set of key attributes needed to indicate relationships, but also any number of other attributes that characterize the relationship.

For example, the associations "Marriage" from the examples 2.1 And 2.4 contain the key attributes "Code_M", "Code_F" and "Husband's personnel number", "Wife's personnel number", as well as clarifying attributes "Certificate number", "Registration date", "Registration_place", "Registry office book entry number", etc. .d.

Characteristic entity (characteristic)

Characteristic entity (characteristic) is a many-to-one or one-to-one relationship between two entities (a special case of an association). The only purpose of a characteristic within the subject area under consideration is to describe or clarify some other entity. The need for them arises due to the fact that the entities of the real world sometimes have multiple-valued properties. A husband can have several wives (example 2.3), a book can have several reissue characteristics (corrected, supplemented, revised, ...), etc.

The existence of a characteristic depends entirely on the entity being characterized: women lose the status of wives if their husband dies.

To describe the characteristic, a new JIM sentence is used, which in the general case has the form:

CHARACTERISTIC (attribute 1, attribute 2, ...)

(LIST OF CHARACTERIZED ENTITIES).

We will also expand the language of ER-diagrams by introducing a trapezoid for the image of the characteristic (Fig. 2.2).

Rice. 2.2. Elements of the Extended ER Diagram Language

Denoting entity or designation

denoting entity or designation is a many-to-one or one-to-one relationship between two entities and differs from a characteristic in that it does not depend on the designated entity.

Example

Consider an example related to the enrollment of employees in various departments of an organization.

In the absence of strict rules (an employee can be simultaneously enrolled in several departments or not enrolled in any department), you must create a description with the association Enrollment:

Employees (Personnel Number, Surname, ...)

Enrollment [Departments M, Employees N]

(Department number, Personnel number, Date of enrollment).

However, provided that each of the employees must be enrolled in one of the departments, you can create a description with the designation Employees:

Departments (Department number, Department name, ...)

Employees (Personnel number, Last name, ..., Department number,

Enrollment date)[Departments]

In this example, the employees have an independent existence (if a department is deleted, it does not follow that the employees of that department should also be deleted). Therefore, they cannot be characteristics of departments and are called designations.

Designations are used to store repeated values of large text attributes: "codifiers" of disciplines studied by students, names of organizations and their departments, lists of goods, etc.

The description of the designation outwardly differs from the description of the characteristic only in that the designated entities are not enclosed in curly brackets, but in square brackets:

LEGEND (attribute 1, attribute 2, ...)[LIST

DESIGNATED ENTITIES].

As a general rule, notations are not treated as full-fledged entities, although this would not lead to any error.

Designations and characteristics are not completely independent entities, since they presuppose the presence of some other entity that will be "designated" or "characterized". However, they are still special cases of an entity and can, of course, have properties, can participate in associations, designations, and have their own (lower level) characteristics. We also emphasize that all instances of a characteristic must necessarily be associated with some instance of the entity being characterized. However, it is allowed that some instances of the characterized entity have no links. True, if it concerns marriages, then the essence of "Husbands" should be replaced by the essence of "Men" (there is no husband without a wife).

Let us now redefine a core entity as an entity that is neither an association, nor a designation, nor a characteristic. Such entities have an independent existence, although they may refer to other entities, as, for example, employees refer to departments.

In conclusion, let's consider an example of building an infological model of the "Food" database, where information about dishes (Fig. 2.3), their daily consumption, the products from which these dishes are prepared, and the suppliers of these products should be stored. The information will be used by the chef and the manager of the small catering establishment, as well as its visitors.

Rice. 2.3. Recipe Example

With the help of these users, the following objects and characteristics of the designed base were identified:

Dishes that require data included in their culinary recipes to describe them: dish number (for example, from a recipe book), dish name, dish type (appetizer, soup, main course, etc.), recipe (dish preparation technology), yield (portion weight), name, calorie content and weight of each product included in the dish.

For each supplier of products: name, address, name of the supplied product, date of delivery and price at the time of delivery.

Daily food intake (consumption): dish, number of servings, date.

Analysis of objects allows you to highlight:

bars of Dishes, Products and Cities;

Associations Composition (associates Dishes with Products) and

Deliveries (associates Suppliers with Products);

designation Suppliers;

characteristics Recipes and Consumption.

The ER diagram of the model is shown in fig. 2.4. and the model in the JIM language has the following form:

Dishes (BL, Dish, View)

Products (PR, Product, Calories)

Suppliers (POS, City, Supplier) [City]

Composition [Meals M, Products N] (BL, PR, Weight (g))

Supplies [Suppliers M, Products N] (POS, PR, Date_P, Price, Weight (kg))

Cities (City, Country)

Recipes (BL, Recipe) (Dishes)

Consumption (BL, Date_R, Portions) (Meals)

In these models, Dish, Product and Supplier are names, and BL, PR and POS are digital codes of dishes, products and organizations supplying these products.

Rice. 2.4. Infological model of the database "Food"

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2.4. About primary and foreign keys

Recall that key or possible key- this is the minimum set of attributes, by the values of which you can uniquely find the required instance of the entity. Minimality means that the exclusion from the set of any attribute does not allow the entity to be identified by the remaining ones. Every entity has at least one possible key. One of them is taken primary key. When choosing a primary key, preference should be given to non-composite keys or keys composed of a minimum number of attributes. It is also inappropriate to use keys with long text values (it is preferable to use integer attributes). So, to identify a student, you can use either a unique record book number, or a set of last name, first name, patronymic, group number, and there may be additional attributes, since it is possible that two students (and more often female students) appear in a group with the same last names, first names and patronymics. It is also bad to use as a key not the number of the dish, but its name, for example, "Appetizer of processed cheese "Friendship" with ham and pickles" or "Hare in sour cream with potato croquettes and red cabbage salad".

It is not allowed for the primary key of the core entity (any attribute participating in the primary key) to have an undefined value. Otherwise, a contradictory situation will arise: a copy of the core entity that does not have individuality, and therefore does not exist, will appear. For the same reasons, it is necessary to ensure uniqueness primary key.

Now oh foreign keys:

If entity C links entities A and B, then it must include foreign keys corresponding to the primary keys of entities A and B.

If entity B denotes entity A, then it must include a foreign key corresponding to entity A's primary key.

In p. 2.3 considered an example where "Employees" stood for "Departments" and included a foreign key "Department Number" corresponding to the primary key of the "Departments" entity.

The relationship between primary and foreign keys of entities is illustrated in fig. 2.5.

Rice. 2.5. Structures: a - associations; b - designations (characteristics)

Here, to designate any of the associated entities (rods, characteristics, designations, or even associations), a new generalizing term "Goal" or "Target Entity" is used.

Thus, when considering the problem of choosing how to represent associations and notations in a database, the main question to be answered is "What are foreign keys?". And further, for each foreign key, three questions must be solved:

1. Can this foreign key take on null values (NULL values)? In other words, can there be some instance of an entity of a given type for which the target entity indicated by the foreign key is unknown? In the case of deliveries, this is probably not possible - a delivery by an unknown supplier or a delivery of an unknown product does not make sense. But in the case of employees, however, this situation could make sense - it is quite possible that an employee is not currently enrolled in any department at all. Note that the answer to this question does not depend on the whim of the database designer, but is determined by the actual course of action adopted in that part of the real world that should be represented in the database in question. Similar remarks are relevant to the issues discussed below.

2. What should happen when you try to DELETE the target entity referenced by the foreign key? For example, when deleting a vendor that has made at least one delivery. There are three possibilities:

3. What should happen when you try to UPDATE the primary key of a target entity referenced by some foreign key? For example, an attempt may be made to update the number of such a supplier for which there is at least one matching supply. For definiteness, we consider this case again in more detail. There are the same three possibilities as for deletion:

CASCADATES	The update operation "cascades" to also update the foreign key in the vendor's shipments.
LIMITED	The primary keys are updated only for those suppliers who have not yet delivered. Otherwise, the update operation is rejected.
INSTALLED	For all deliveries from such a provider, the NULL foreign key is set to null and then the provider's primary key is updated. This possibility is, of course, not applicable if the given foreign key should not contain NULL values.

Thus, for each foreign key in a design, the database designer must specify not only the field or combination of fields that make up that foreign key and the target table that is identified by that key, but also the answers to the above three questions (the three constraints that relate to to that foreign key).

Finally, about the characteristics - designating entities, the existence of which depends on the type of designated entities. The designation is represented by a foreign key in the table corresponding to this characteristic. But the three foreign key constraints discussed above for this case must be specified as follows:

NULL values are not allowed

DELETE FROM (target) CASCADATED

UPDATE (target primary key) CASCADATED

These specifications represent the dependency on the existence of characteristic entities.

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2.5. Integrity Constraints

The concept of data integrity

Integrity(from the English integrity - integrity, integrity, safety, integrity) - is understood as the correctness of the data at any time. But this goal can only be achieved within certain limits: the DBMS cannot control the correctness of each individual value entered into the database (although each value can be checked for plausibility). For example, it is not possible to detect that the input value 5 (representing the number of the day of the week) should actually be equal to 3. On the other hand, the value 9 would clearly be erroneous and the DBMS should reject it. However, to do so, it must be told that the weekday numbers must belong to the set (1,2,3,4,5,6,7).

Maintaining the integrity of a database can be thought of as protecting data from erroneous modifications or destruction (not to be confused with illegal modifications and destruction, which is a security issue). Modern DBMSs have a number of tools for maintaining integrity (as well as tools for maintaining security).

Types of integrity

There are three groups of integrity rules:

Entity integrity.

User defined integrity.

In p. 2.4 the rationale for two integrity rules common to all relational databases was considered.

It is not allowed for any attribute participating in the primary key to have an undefined value.

The foreign key value must either:

be equal to the target's primary key value;

be completely indeterminate, i.e. each value of an attribute participating in a foreign key must be null.

For any particular database, there are a number of additional specific rules that apply to it alone and are determined by the developer. Most often controlled:

the uniqueness of certain attributes,

range of values (examination score from 2 to 5),

belonging to a set of values (sex "M" or "F").

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2.6. About building an infological model

Introduction

The reader, who has only read the material of this and the previous chapters, will not be able to correctly perceive and evaluate those tips and recommendations for building a good infological model, which have been formed over decades by the largest specialists in the field of data processing. To do this, you must at least study the following materials. Ideally, it is necessary that the reader has previously implemented at least one information system project, proposed it to real users, and has been a database and application administrator for so long as to realize at least a fraction of the problems that arise due to an insufficiently thought out project. The experience of the author and all information systems specialists he knows shows that any theoretical recommendations are taken seriously only after several unsuccessful attempts to revive unsuccessfully designed systems. (Although there are some designers who continue to believe they can revive a dying project by changing programs rather than the database infological model.)

Requirements for the database by the administrator and application programmer

Indeed, to determine the list and structure of stored data, it is necessary to collect information about real and potential applications, as well as about database users, and when building an infological model, one should only care about the reliability of storing this data, completely forgetting about the applications and users for which the database is being created. data.

This is due to the completely different database requirements of application programmers and database administrators. The former would like to have in one place (for example, in one table) all the data they need to implement a request from an application program or from a terminal. The latter take care of excluding possible distortions of the stored data when new information is entered into the database and the existing information is updated or deleted. To do this, they remove duplicates and unwanted functional relationships between attributes from the database, splitting the database into many small tables (see p. 4.6 ). Since many years of world experience in the use of information systems built on the basis of databases shows that design flaws cannot be eliminated by any tricks in application programs, experienced designers do not allow themselves to meet the needs of application programmers (even when they themselves are such).

clearly distinguish between such concepts as a request for data and data maintenance (input, change and deletion);

remember that, as a rule, the database is the information basis of not one, but several applications, some of which will appear in the future;

a bad database design cannot be fixed with any (even the most sophisticated) applications.

When looking at relationships between domain entities in detail, you can see that they have different properties in relation to each other. This is the reason for dividing entities into types. A single, well-established point of view on the classification of entity types has not yet developed. As a rule, it is customary to distinguish the following types of entities.

Independent entity An entity whose existence does not depend on the existence of other entities. It does not need additional information taken from another entity to identify a unique instance. Therefore, the primary key of an independent entity does not include the primary keys of other entities.

Example 4: subject area - educational process. Each student is characterized by academic performance - a set of marks received in the session. The ASSESSMENT entity is dependent, since for its identification it requires a reference to the student to whom it was delivered. Therefore, its primary key must include the primary key of the STUDENT entity.

Independent entities can have foreign keys, i.e. refer to other entities. If an independent entity has no foreign keys (that is, it is not related to other entities in any way), then it is called core entity or rod. On JIM, the core entity is written as follows:

ENTITY NAME (attribute1, attribute2, ....),

where there are no foreign keys among the attributes, and key attributes are underlined. On ER diagrams, core entities are represented by rectangles.

If an independent entity has foreign keys (that is, it is related to other entities), then it is called denoting entity or designation. The relationship to the designated entity can be one-to-one or many-to-one. The denoting entity is described as follows:

DESIGNATION (attribute1, attribute2,..)[LIST OF DESIGNATED ENTITIES],

where among the attributes there are foreign keys. In ER diagrams, denoting entities are depicted as parallelograms.

Example 5: subject area - distribution of employees by departments.

THE DEPARTMENT ( Department number, department name, …)

EMPLOYEE ( Personnel Number, Surname, …, Department number) [DEPARTMENT].

Here the entity EMPLOYEE is the designation of the entity DEPARTMENT due to the presence of a foreign key Department number; at the same time, the EMPLOYEE entity retains an independent existence: if this attribute is not filled, then this means that the employee is not enrolled in any department.

Characteristic entity or a characteristic is a dependent entity whose existence is wholly dependent on the characterized entity with which it is related in a one-to-one or many-to-one relationship. The characteristic entity is described as follows:

CHARACTERISTIC (attribute1, attribute2,..) (LIST OF CHARACTERIZED ENTITIES),

in this case, the primary key of the characterized entity is included in the primary key of the characteristic or is its mandatory foreign key. In the previous example, an employee could be part of a department, or could have an independent existence, so the corresponding entity was defined as a designation. But in any institution, the employee must be a member of a department, this will change the status of the EMPLOYEE entity to a DEPARTMENT characteristic.

Example 6: subject area - student performance.

STUDENT (Last name, Record number, ...)

PROGRESS ( Record number, Discipline, Grade) (STUDENT)

Here, the primary key The number of the gradebook of the characterized entity STUDENT is included in the primary key of the characteristic PROGRESS. An empty value of this attribute is invalid, since in this case it becomes undefined which student's performance this line characterizes, that is, the concept of academic performance loses its meaning.

On ER diagrams, characteristic entities are represented by trapezoids.

Associative entity or an association is a many-to-many relationship between two or more entities. Often introduced into a model to split a many-to-many relationship into two one-to-many relationships. At the same time, they are considered as full-fledged entities. This means that associations can participate in other associations and designations as core entities, they can have attributes, and so on. A characteristic entity can be viewed as a special case of an association. Associations are described as follows:

ASSOCIATION [ENTITY S1, ENTITY S2, ...] (attribute1, attribute2, ...),

where S1, S2, etc. – the degree of connection with the specified entity. In ER diagrams, associations are depicted using hexagons.

Example 7: formal description of the ILM: subject area - the educational process at the university (option). The model should reflect the existence of students, study groups, teachers and scheduled classes. The model should also reflect student performance from the previous session.

Let's single out the core entities in the subject area. This is GROUP and TEACHER:

GROUP ( Group code, Department, Composition, Warden)

TEACHER ( Surname

The STUDENT entity refers to the GROUP entity, therefore it is its designation:

STUDENT (Last name, Record number, Group number) [GROUP]

A student's performance is his characteristic:

PROGRESS ( Record number, Discipline, Grade) (STUDENT)

Finally, the entity ACTIVITY is an association of the entities GROUP and TEACHER; in addition, it has its own attributes - discipline, time and place:

Group code, Last name of the teacher, Discipline,

The ER-diagram of the subject area has the form (Fig. 1):

Now let's introduce a new entity into the model - DISCIPLINE. For simplicity, we will assume that it has a single characteristic - Name and is designed to encode long text names of disciplines. In all other entities, we will replace the name with the discipline code and establish links with the new entity. This small change will change the status of the STUDY entity - it will turn into an association; the entity OCCUPATION from an association of two entities will become an association of three entities:

GROUP ( Group code, Department, Composition, Warden)

TEACHER ( Surname, Department, Position, Academic degree)

STUDENT (Last name, Record number, Group number) [GROUP]

PROGRESS [STUDENT M, DISCIPLINE N] ( Record number, Discipline code, Grade)

LESSON [GROUP M, TEACHER N] ( Group code, Last name of the teacher, discipline code, Week, Day, Couple, Audience).

This example demonstrates the fact that a characteristic is a special case of an association that has fewer than two dependent relationships.

Basic concepts of the relational data model

The main data structure in the model is the relation, which is why the model is called relational (from the English relation - relation). From the user's point of view, the notion of a relation can be synonymous with the notion of a table, i.e. in the relational model, data is stored as a set of interconnected tables. Let's look at a few definitions.

Essence - this is any distinguishable, different, real or imaginary object, information about which will be stored in the database. The concept of entity refers to the fundamental concepts of database theory and is used to model a class of similar objects. Entities (classes of objects) and instances of objects are singled out in the framework of the subject area under consideration in the process of its analysis. For example, for a chemical database, you can select the entity (class of objects) CHEMICAL ELEMENT and instances of objects - individual chemical elements, for example, HYDROGEN or HELIUM. Each object class has its own unique name within the subject area, which makes it possible to distinguish one entity from another.

Each object related to this entity has its own set attributes - characteristics that determine the properties of a given class of objects and allow you to uniquely identify an object and distinguish it from others. All instances of objects belonging to the same entity have the same set of attributes and differ only in attribute values. So, for example, the entity CHEMICAL ELEMENT has such attributes as SERIAL NUMBER, NAME, SYMBOL, ATOMIC MASS and a number of others. Each attribute of an entity has its own unique name, but different entities can have the same attribute names. For example, the attribute ORDINAL NUMBER can be for any ordered set of objects.

Domain is the range of values that the attribute can take. The domain can be specified by enumeration of possible values or in an abstract form. For example, the DAY of the WEEK attribute can take values from a limited set ("Mon", "Tue", "Wed", "Thursday", "Fri", "Sat", "Sun"), and the attribute SEQUENCE NUMBER belongs to the domain of positive integers. numbers. Different attributes of an object can take values from the same domain, for example, ORDINAL NUMBER and ATOMIC MASS belong to the domain of positive integers.

Thus, each entity corresponds to a table. Entity attributes are column headings, and table rows are called records and contain attribute values for different object instances.

Entity types. Database design with ERwin Any entity different from other entities

General concept

What do entities do with a person?

Subtle human bodies and entities

Astral entities: types

Lyarvy

Incubus and succubus

Elementors

Astral entities: classification according to L. G. Puchko

Prevention

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Similar Documents

2.3. Entity classification

Three classes of entities

Rod entity (pivot)

Associative entity (association)

Characteristic entity (characteristic)

Denoting entity or designation

Example

2.4. About primary and foreign keys

2.5. Integrity Constraints

The concept of data integrity

Types of integrity

2.6. About building an infological model

Introduction

Requirements for the database by the administrator and application programmer

Basic concepts of the relational data model