Can someone tell me the easy way to explain the differentiation between an entity and a table in database?
Entity is a logical concept of relational database model. And table is used to express it, but there is a slight difference. Table expresses not only entities, but also relations.
For example, assume that you want to make a database of projects and employees of a company. Entity is a unit of information that has meanings by itself. In this case, there will be two entities - "Project" and "Employee". Each entity has its own attributes.
In relational DB model, there is another idea, 'relation'. If employees participate in several projects, then we can say that there is a relation with a name 'works_on'.
Sometimes, relation can have its own attribute. In this case, 'works_on' relation can have attribute 'start_date' and so on. And if this relation is M:N relation(Like this case: In project 1, there are 5 employees. Employee A works on two projects.), then you have to make an extra table to express this relation. (If you don't make an extra table when the relation is M:N, then you have to insert too many duplicated rows into both 'Project' and 'Employee' table.)
CREATE TABLE works_on(
employee,
project_id,
start_date
)
An entity resides in a table, it is a single set of information, i.e: if you have a database of employees, then an employee is an entity. A table is a group of fields with certain parameters.
Basically everything is stored in a table, entities goes into tables.
In a relational database the concept is the same. An entity is a table.
In OOP (Oriented Object Programming) there is a nice article in Oracle docs:
In general terms, entity objects encapsulate the business policy and
data for
the logical structure of the business, such as product lines,
departments, sales, and regions
business documents, such as invoices, change orders, and service
requests
physical items, such as warehouses, employees, and equipment
Another way of looking at it is that an entity object stores the
business logic and column information for a database table (or view,
synonym, or snapshot). An entity object caches data from a database
and provides an object-oriented representation of it.
Depending on how you want to work, you can create entity objects from
existing database tables (reverse generation) or define entity objects
and use them to create database tables (forward generation).
There is little difference between an entity and a table, but first we should define the concepts of “tuple” and “attribute”. I want to express those concepts with a table.
Here is an example of a table. As you can see it has some columns. Each column represents an attribute and each line represents a tuple(row). This is the employee table. In this example, the table is being used for representing an entity which is employee. However, if we create a new table named superior to specify the superiority relationship between those employees, it would be a table that represents a relation. In summary, we can use tables for representing both the entities and relations so entities and tables are not the same.
Related
Take person for example
Does the Person entity refer to Jon Doe as a single person or the people as a whole ?
For example in a table in a database, is there a Table which holds a set of entities or an Entity which holds some records ?
Apart from database, how about data modelling?
In the Entity Relationship model, entity refers to a thing which
can be distinctly identified. A specific person, company, or event is an example of an entity. A population of entities of the same type is called an entity set. When an entity set is identified and described via attributes, we get an entity relation, which can be represented as a table in a database.
Note that tables are also used to represent relationship relations, so it's not correct to equate tables with entity sets or relations. It's also not correct to equate a row with a single entity - rather, a row represents a fact about one or more entities.
I have the following ERD:
ER Diagram
I need to convert this into relational schema but I have some doubts on where the attribute "since" would belong. I know how 1-M relationships work but I am confused by the attribute since. Does a new table Company-Employee need to be created where it would hold the Company id, emp id and since?
Please let me know thank you.
As you have diagrammed it, the Since attribute describes the relationship, not the employee or the company. There's nothing wrong with that, although some experts maintain that attributes must describe entities and not relationships.
When you go to design a relational schema, both entities and relationships become relations (or, as I prefer, "tables"). Your only question seems to be whether there has to be a junction table between the Employee table and the Company table.
You can get away with just two tables. You can stuff both the CompanyId and the Since columns into the Employee table, because they will be single valued. There may be other considerations that lead you to decompose the two tables, but you don't have to do it just to represent the data.
While studying for my IT exam I came across the following sentence:
"A collection of fields that store information about a certain entity, is a record. A record is a whole row of fields."
..but I have always thought that the correct term for an "object" in a database is an "entity".
So is the correct term an "entity" or a "record"? Or are they the same?
In that sentence, entity doesn't refer to anything in the database. It's using entity to refer to a conceptual object, whatever thing in the real world the database record represents. For instance, if you have an inventory database, each row stands for a product in the warehouse, and that's the entity.
An entity is defined as “something that exists as a particular and
discrete unit.” In terms of identity management, an entity is the
logical relationship between two or more records. [...] An entity is
also called a “linkage set.” There can be an unlimited number of
records in an entity or linkage set.
Source
Along these lines, an entity can be a set of records in a table or even across different tables.
I would say that an entity concept is physicalised by 1 or more tables e.g.
a product concept might be encapsulated entirely in 1 table
a person concept might be spread across several tables, for example due to normalisation - all information relating to a person might not exist in the same table.
Every time I program I recognize this relationship between classes and tables, or am I imagining it.
You can have a class per database table or a table per class i.e. :
tables: customer, products, order.
classes: customer, products, order, may have methods such as addRecord, deleteRecord, updateRecord.
what is this called? Object-Relational? I am not a DBA.
It all depends on the type of database you're using. If you're using an object oriented database (OODB), then there is no relationship, as the objects and the persisted data are the same thing. For example, if you have a Customer class, and you save it in an OODB, then that instance of the customer is what is stored in the DB.
If you are using a relational database, then the class instances, and the persisted representation of them in the DB, can be the same thing, but many times they aren't. This is because most folks use normalization to represent their data in an efficient way (in a relational DB). This means, instead of having a table per class, you can have a class represented by more than one table. In the Customer example, the tables might now be Customer (with Name, date of birth, and other properties), and Order (with order pointing to products in yet another table). The reason for this has to do with cardinality, and the ability for Customers to have more than one order. When your business logic needs this information from the DB, the data access layer's job is to map the data (called ORM) from the DB into your classes.
If you are using yet another type of DB, then there will be a different relationship between the classes (domain model) and what's persisted in the DB.
But, as far as having a name for this relationship? No, there is no name.
In additon to Bob's answer, the following.
In object modeling, the relationship between classes and subclasses is taken care of by inheritance, and object modelers know how to use inheritance to good advantage. The relational data model and by extension the SQL databases do not implement inheritance for you. You have to design tables to give you some of the same results.
In ER (Entity-Relationship) modeling, the corresponding concept is called generalization/specialization. This tells you how to model a class/subclass relationship, but it doesn't tell you how to design the tables when you go to build your database.
There are three techniques that are pretty well understood that can be really helpful when dealing with classes and subclasses. Here are their tags: single-table-inheritance class-table-inheritance shared-primary-key. Unfortunately, many tutorials on database design never cover these techniques. They can be enormously useful to people who know object modeling and want to come up to speed on relational modeling.
I just started with the Entity Framework and started to design the model first. So in my model there is a Person who can have a PrivateTelephone, so I created an 0..1 to 1 association. As the picture below shows.
So far so good. But when I generate the database the [PrivateTelephone] is set to NOT NULL. Why can't this be just NULL?
It is becauser your relations are defined in reverse order. You should have 1 on Person and 0..1 on Telecom to specify that Person is the principal which can have one or zero phones. In your mapping you say that Telecom is principal which can have one or zero persons but person must have Telecom. It will also lead to reverse problem due to your incorrect mapping. You have six one-to-one relations to Telecom but if you reverse them as you demand you will say that all six relations (all six FKs in Telecom) will be NOT NULL = each record will have to participate in all six relations.
One-to-one relation is very special and should be used rarely. You should instead have one-to-many relation from Person to Telecom with a new column in Telecom specifying the type.
When using one-to-one relation you must have FK in dependent table configured with unique index. EF doesn't support unique indices so when you model one-to-one relation in model first it is still one-to-many in database and if database is used by another application it can break your application.
Also avoid unnecessary inheritance. Do you need Person as separate entity? = is there any instance which is only person and not employee? Are there more derived types from person? If not you don't need person and if yes it still doesn't mean that base person is a good idea. The same is true with employee. Inheritance has its own rules in EF and when using model first it will by default creates TPT inheritance = the worst one because it results in very complex and slow database queries.