What is an entity in
database management system
An entity is a
person,place,thing or event for which data is collected and maintained.
for example...
a library system may contain data about different entities like BOOK and MEMBER.
A college system may include entities like STUDENT,TEACHER and CLASS.
for example...
a library system may contain data about different entities like BOOK and MEMBER.
A college system may include entities like STUDENT,TEACHER and CLASS.
An entity is a distinct or
discrete things or objects which helps us to make a efficient data management
system providing the relation between the entities. For example - An office
could have these entities like- Employees, Department, Salary.
A record, table or index.
an entity is close in
meaning to object as it is used in object-oriented
programming .
2) In relation to a database , an entity is a single person, place, or
thing about which data can be stored.
3) In data modeling (a first step in the creation of a database),
an entity is some unit of data that can be classified and have stated
relationships to other entities.
Entity Relationship Diagrams
Entity Relationship diagrams (also known as E-R or ER diagrams) provide database designers with a valuable tool for modeling the relationships between database entities in a clear, precise format. This industry standard approach uses a series of block shapes and lines to describe the structure of a database in a manner understandable to all database professionals. Many database software packages, including Microsoft Access, SQL Server, andOracle, provide automated methods to quickly create E-R diagrams from existing databases.
In this article, we provide an overview of E-R diagramming techniques to help you read, modify or create your own data models.
Entities
In a database model, each
object that you wish to track in the database is known as an entity. Normally,
each entity is stored in a database table and every instance of an entity corresponds
to a row in that table. In an ER diagram, each entity is depicted as a
rectangular box with the name of the entity contained within it.
For example, a database containing information about individual people would likely have an entity called Person. This would correspond to a table with the same name in the database and every person tracked in the database would be an instance of that Person entity and have a corresponding row in the Person table. Database designers creating an E-R diagram would draw the Person entity using a shape similar to this:
They would then repeat the process to create a rectangular box for each entity in the data model.
For example, a database containing information about individual people would likely have an entity called Person. This would correspond to a table with the same name in the database and every person tracked in the database would be an instance of that Person entity and have a corresponding row in the Person table. Database designers creating an E-R diagram would draw the Person entity using a shape similar to this:
They would then repeat the process to create a rectangular box for each entity in the data model.
Attributes
Of course, tracking
entities alone is not sufficient to develop a data model. Databases contain
information about each entity. This information is tracked in individual fields
known as attributes, which normally correspond to the columns of a database table.
For example, the Person entity might have attributes corresponding to the person's first and last name, date of birth, and a unique person identifier. Each of these attributes is depicted in an E-R diagram as an oval, as shown in the figure below:
Notice that the text in the attribute ovals is formatted slightly differently. There are two text formatting features used to convey additional information about an entity's attributes. First, some fields are displayed in a boldface font. These are required fields, similar to the NOT NULL database constraint. Each instance of an entity must contain information in the FirstName, LastName and PersonID attributes. Also, one attribute is underlined, indicating that it serves as the database's primary key. In this example, PersonID serves as the primary key.
Using this format can be somewhat cumbersome in a diagram containing information about entities with many attributes. Therefore, many database designers prefer to use an alternate format that lists an entity's attributes in tabular form under the name of the entity. Here's a depiction of the Person entity in that format:
That covers the "Entity" part of Entity-Relationship diagrams. Now let's take a look at displaying data relationships.
For example, the Person entity might have attributes corresponding to the person's first and last name, date of birth, and a unique person identifier. Each of these attributes is depicted in an E-R diagram as an oval, as shown in the figure below:
Notice that the text in the attribute ovals is formatted slightly differently. There are two text formatting features used to convey additional information about an entity's attributes. First, some fields are displayed in a boldface font. These are required fields, similar to the NOT NULL database constraint. Each instance of an entity must contain information in the FirstName, LastName and PersonID attributes. Also, one attribute is underlined, indicating that it serves as the database's primary key. In this example, PersonID serves as the primary key.
Using this format can be somewhat cumbersome in a diagram containing information about entities with many attributes. Therefore, many database designers prefer to use an alternate format that lists an entity's attributes in tabular form under the name of the entity. Here's a depiction of the Person entity in that format:
That covers the "Entity" part of Entity-Relationship diagrams. Now let's take a look at displaying data relationships.
Relationships and Cardinality
The power of the E-R
diagram lies in its ability to accurately display information about the
relationships between entities. For example, we might track information in our
database about the city where each person lives. Information about the city
itself is tracked within a City entity and a relationship is used to tie
together Person and City instances.
Relationships are normally given names that are verbs, while attributes and entities are named after nouns. This convention makes it easy to express relationships. For example, if we name our Person/City relationship "Lives In", we can string them together to say "A person lives in a city." We express relationships in E-R diagrams by drawing a line between the related entities and placing a diamond shape that contains the relationship name in the middle of the line. Here's how our Person/City relationship would look:
Notice that there are some additional shapes on
the line. The double hashed line appearing just to the left of the City entity
indicates that this part of the relationship has a cardinality of 1. On the
other hand, the crow's foot symbol to the right of the Person entity indicates
that this part of the relationship has a cardinality of "many".
Stated more plainly, each person may live in only one city, while a city may
contain many people.
Those are the basics of Entity-Relationship diagrams. You should now have the information you need to create basic diagrams for your databases.
Relationships are normally given names that are verbs, while attributes and entities are named after nouns. This convention makes it easy to express relationships. For example, if we name our Person/City relationship "Lives In", we can string them together to say "A person lives in a city." We express relationships in E-R diagrams by drawing a line between the related entities and placing a diamond shape that contains the relationship name in the middle of the line. Here's how our Person/City relationship would look:
Notice that there are some additional shapes on
the line. The double hashed line appearing just to the left of the City entity
indicates that this part of the relationship has a cardinality of 1. On the
other hand, the crow's foot symbol to the right of the Person entity indicates
that this part of the relationship has a cardinality of "many".
Stated more plainly, each person may live in only one city, while a city may
contain many people.Those are the basics of Entity-Relationship diagrams. You should now have the information you need to create basic diagrams for your databases.
Definition: An entity-relationship (ER)
diagram is a specialized graphic that illustrates the relationships between entities in a database.
ER diagrams often use symbols to represent three different types of
information. Boxes are commonly used to represent entities. Diamonds are
normally used to represent relationships and ovals are used to represent
attributes.
For more information, see our full feature story on Entity-Relationship Diagrams.
For more information, see our full feature story on Entity-Relationship Diagrams.
Also
Known As: ER
Diagram, E-R Diagram, entity-relationship model
Examples: Consider the example of a
database that contains information on the residents of a city. The ER digram
shown in the image above contains two entities -- people and cities. There is a
single "Lives In" relationship. Each person lives in only one city,
but each city can house many people.
Differentiating between domain, model, and entity with respect to MVC
The terms are a bit vague I agree. I would use domain to refer
to the business area you are dealing with. Like banking or insurance or what
not. Then you have domain models. These are the things you deal with in that
business domain, like for domain of banking you have accounts, customers,
transfers etc. I would use term entity for referencing the class/POJO or the
persisted / concrete version of a model.
What probably confuses you here is that in the term MVC, the model is a concrete thing, but it references the
data model used to represent some presentation in a web GUI so don't get that
mixed in with the above explanation.
The terms you are confused about are: "domain
objects", "domain entities" and "model
objects". While usually used interchangeably, the domain entities and
model object can also be instances of active record patter
(basically: domain objects with added storage logic).
In ordinary domain object there is no storage logic. It gets
handled by data mappers.
The term "model objects" comes from Fowler's books (read PoEAA for
more details), and, IMHO, is part of the confusions MVC, because the entire
model is an application layer (MVC consists of it and presentation layer),
which contains those "model objects", which are usually dealt with by services (in
that image, the model layer is all three concentric circles together).
I much prefer to use
"domain object" term instead.
The term "domain
entity" (or "entity object") is usually used when author implies
that the object is a direct representation of a storage structure (more often -
a database table). These are also almost always implementations of active
record.
entity-relationship model (diagram)
(n.) Also
called an entity-relationship
(ER) diagram, a graphical representation of entities and their relationships
to each other, typically used in computing in regard to the organization of datawithin databases or
information systems. An entity is a piece of data-an object or
concept about which data is stored.
A relationship is how the data is shared between entities. There
are three types of relationships between entities:
1. One-to-One
One instance of an entity (A) is associated with one other
instance of another entity (B). For example, in a database of employees, each
employee name (A) is associated with only one social security number (B).
NuoDB: A
Truly Distributed Database
2. One-to-Many
One instance of an entity (A) is associated with zero, one or
many instances of another entity (B), but for one instance of entity B there is
only one instance of entity A. For example, for a company with all employees
working in one building, the building name (A) is associated with many
different employees (B), but those employees all share the same singular
association with entity A.
3. Many-to-Many
One instance of an entity (A) is associated with one, zero or
many instances of another entity (B), and one instance of entity B is
associated with one, zero or many instances of entity A. For example, for a
company in which all of its employees work on multiple projects, each instance
of an employee (A) is associated with many instances of a project (B), and at
the same time, each instance of a project (B) has multiple employees (A)
associated with it.
Data modeling
Data modeling is a process used to define and analyze data requirements needed
to support the business processes within the scope of corresponding
information systems in organizations. Therefore, the process of data modeling
involves professional data modelers working closely with business stakeholders,
as well as potential users of the information system.
There
are three different types of data models produced while progressing from
requirements to the actual database to be used for the information system.[2] The
data requirements are initially recorded as a conceptual data model which is essentially a set of technology
independent specifications about the data and is used to discuss initial
requirements with the business stakeholders. The conceptual model is then translated into a logical data model,
which documents structures of the data that can be implemented in databases.
Implementation of one conceptual data model may require multiple logical data
models. The last step in data modeling is transforming the logical data model
to a physical data model that organizes the data into tables, and
accounts for access, performance and storage details. Data modeling defines not
just data elements, but also their structures and the relationships between
them.[3
The data modeling
process. The figure illustrates the way data models are developed and used
today. A conceptual data model is developed based
on the data requirements for the application that is being developed,
perhaps in the context of an activity model.
The data model will normally consist of entity types, attributes,
relationships, integrity rules, and the definitions of those objects. This is
then used as the start point for interface or database design.[1]
2) what is the purpose of
using LINQ in asp.net?
LINQ Providers are a set of classes that takes a LINQ query and dynamically generates a method that executes an equivalent query against a specific data source.
3) Advantage and Disadvantage of using LINQ?
It allows you to query collections like arrays, enumerable classes etc in the native language of your application, like VB or C# in much the same way as you would query a database using SQL. LINQ to SQL, which is what most of the current fuss is about, is an Object/Relational Mapper (O/RM) built in to Visual Studio. It automatically generates entity classes based on the database schema, so it helps speed up development of n-layer arhitecture applications.
LINQ Providers are a set of classes that takes a LINQ query and dynamically generates a method that executes an equivalent query against a specific data source.
3) Advantage and Disadvantage of using LINQ?
It allows you to query collections like arrays, enumerable classes etc in the native language of your application, like VB or C# in much the same way as you would query a database using SQL. LINQ to SQL, which is what most of the current fuss is about, is an Object/Relational Mapper (O/RM) built in to Visual Studio. It automatically generates entity classes based on the database schema, so it helps speed up development of n-layer arhitecture applications.
biggest disadvantage is we cannot use linq for Oracle other database
engines except sql
biggest advantage is makes it possible for developer to use database entities without knowledge of any db language like sql
biggest advantage is makes it possible for developer to use database entities without knowledge of any db language like sql
In
ADO.NET you are fetching your data as DataSet or DataReader where you have to
Iterate & put those values manually in your object. But, Linq
fetches your data as Object. So you need not to fill your DataObjects using
your code. Rest of all the ADO.NET features are available in Linq also.
Advantages
--------------
So querying data would be easy.
It allows you to query collections like arrays, enumerable classes etc in the native language of your application, like VB or C#
For example
string[] cities = { "London", "Amsterdam", "San Francisco", "Las Vegas",
Advantages
--------------
So querying data would be easy.
It allows you to query collections like arrays, enumerable classes etc in the native language of your application, like VB or C#
For example
string[] cities = { "London", "Amsterdam", "San Francisco", "Las Vegas",
"Boston", "Raleigh", "Chicago", "Charlestown",
"Helsinki", "Nice",
"Dublin" };
GridView1.DataSource = from city in cities
where city.Length > 4
orderby city
select city.ToUpper();
GridView1.DataBind();
Disadvantages---- Time Consuming
let me explain a real time scenario that I faced. In my company for the project I worked we used EntityFramework
and query the database using LInq query.
I had a scenario where I joined 5 tables you how much time it took, It Consumed around 5 to 6 seconds to execute the query itself compared to ADO.Net which took around 500 milliseconds.
So to overcome this we had to change most of the queries to SQL by using method ExecuteStoreQuery and in some cases usingCompiledQuery.
If you are familiar with SQL programming, you know that you always do the work twice while you develop such applications. Firstly you define your model on the database by creating a lot of tables and setting foreign keys between them. Then you write the code to access these tables, place queries and change values. Your logical entities are spread over several tables, you have to join them manually and maintain consistency (almost) manually. This have been the building block of the so called client/server applications since the middle of the 90s. It is implemented in the Microsoft .NET Framework as ADO.NET and deals with classes like
DataSet, DataTable, DataRow and DataView.
Object Relational Mapping (ORM) came into play to access the database with objects. It acts as a layer between the programming language or environment on one hand and the underlying database on the other hand. As a result of this abstraction, the programming of business applications is much faster and the code is more maintainable. ORM Frameworks implement several ways to speed up the development process and increase maintainability. In general, they allow to access the database with objects that reflect entities in an (almost) natural way. Many of them allow to put LINQ queries against the database to get entities. And some of them decouple the business logic completely from the underlying database.
Two of the best known ORM frameworks are NHibernate (.NET) and the ADO.NET Entity Framework. Both of them have only a partial LINQ implementation and they are not known to be the fastest on the racetrack. In this article, I want to introduce DataObjects.Net from x-tensive. DataObjects.net is an ORM with a very high abstraction, an almost complete LINQ implementation and a really good performance.
Three-tier .NET Application Utilizing Three ORM Technologieshttp://www.codeproject.com/Articles/519792/MagicplusWand-aplusObject-relationalplusmapping?display=Print
I would like to discuss today the advantages and disadvantages of using ORM (such as ADO.NET).
Advantages:
- Speeds-up Development - eliminates the need for repetitive SQL code.
- Reduces Development Time.
- Reduces Development Costs.
- Overcomes vendor specific SQL differences - the ORM knows how to write vendor specific SQL so you don't have to.
Disadvantages:
- Loss in developer productivity whilst they learn to program with ORM.
- Developers lose understanding of what the code is actually doing - the developer is more in control using SQL.
- ORM has a tendency to be slow.
- ORM fail to compete against SQL queries for complex queries.
In summary, I believe that the advantages of using an ORM (mainly the reduced time taken to perform repetitive tasks) are far outweighed by the disadvantages of ORM e.g. it's difficulty to get to grips with.
"ORM fail to compete against SQL queries for complex queries."
- Well both LINQ-SQL and Entity Framework Allow complex queries and even translation of SQL query results into objects.
"Developers loose understanding of what the code is actually doing - the developer is more in control using SQL."
- Not really, if you know what you are doing. SQL profiler is enough to see what the translated SQL queries are.
"ORM has a tendency to be slow."
- Yes, but delay loading and some smart options can make it almost as fast.
"Loss in developer productivity whilst they learn to program with ORM. "
- Hibernate and the Entity Framework might take time to learn but in the long run they will save time in development. LINQ-SQL on the other hand has little to no learning curve involved.
I say, use ORM but keep this in mind.
- Design your queries and write code that will result in the least number of roundtrips with the server. It's the overhead taken for the roundtrip that takes up time.
- Read about the experiences other people have had with the selected ORM before you dig in too deep.
- Always compare your queries with the actual ones being executed in SQL server profiler.
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What is Object Relational mapping? In a nutshell, it is a technique to provide automatic retrieval and persistence of data. Usually this involves using a configuration file, often in XML, to map fields in your custom coded objects to fields and tables in a database. In this way the OR Mapper can then take ownership of the best way to retrieve, update, insert and delete data and you can focus on building the application and not mess with the plumbing.
Most professional developers use third-party components at some time and OR mapping products are an example of the acceptance and reliance on these types of components. The reality of our work is that almost every application has to touch a database and this data access work is error prone, tedious and honestly, just plain boring after a while. This type of work lends itself to mistakes. I am reminded of the classic software developers axiom, “if you can reduce risk, then do so.” This is one reason to use an OR mapper.
Another OR mapper advantage is the speed of development. Using an OR mapper can drastically reduced the amount of code that needs to be written and debugged, plus it can quicken how you develop. If you are mapping data directly to your objects, essentially making them entity objects, there is not data conversion between Datasets, data readers, structs or anything else. This reduces the complexity and makes working with your data essentially the same as working with your objects. Now your focus is on your domain model and not how ADO.NET is doing this or that task. Please note that I am not saying that understanding ADO.NET is not important – you should understand these concepts to be a productive and knowledgeable developer – but now that you understand the core concepts you can step beyond it and really take advantage of this knowledge.
True OR mappers, which I consider different than code generators, do all their database access dynamically. So, they are not generating files with code in them, and they are not using stored procedures. In the .NET world, the mapper will do all its database access using parameterized queries, which as controversial as it may sound, are just as fast as stored procedures. Thus, you now have the flexibility of dynamically generated SQL with the speed of a stored procedure.
Use of an OR mapper also provides a level of abstraction between your code libraries and the data source. Let us say that you are using MS Access as your application data source and you are using an OR mapper. You can now easily switch from MS Access to SQL Server 2005 without too much trouble because you do not have anything hard coded into your application that ties you to a platform. Extrapolate this out and you can migrate from Oracle to SQL Server or Access to MySQL, or really any way you wish.
The biggest concept to understand here is that the OR Mapper is providing flexibility. Flexibility in how you write your objects, flexibility in how you spend your development time and flexibility in what your data source is. And of course, they are fast at the same time.
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| Different OR Mappers | |
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There are many different OR mappers available. The three products that I consider when thinking about the object relational mapping category are NHibernate, LLBL Gen Pro, RapTier and Wilson ORM.
NHibernate is the open source port of the popular OR mapper Hibernate for java. I am not a java developer so I can not speak to how good of a port it is. I have not used NHibernate myself, mainly because I have found the documentation of the product to be poor so far. There is a very active community around this product, but for my project I did not want to participate in their project, I wanted to focus on building mine. I have read and heard of success stories from colleagues along with their accompanying horror stories with NHibernate. I do consider NHibernate to be a true mapper because it does not generate code for the developer to manage. It does all its work dynamically once configured.
The second mapper is LLBL Gen Pro, which comes with its own environment for mapping objects with database tables and fields. This product does generate code files, so it is not something that I wanted to use this time around.
The third product is called RapTier. I have used this on a couple different projects earlier this millennium with success. RapTier does create code files though, so you have to maintain those and is something I would not consider doing now. But at the time, I experienced this product to be a great time saver because it allowed me to focus on the application and not the data access. The company also provided good support and an aggressive price.
The last product is WilsonORMapper and is ultimately what I decided to use on this project.
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What is Object-relational mapping?
Wikipedia says “Object-relational mapping (ORM, O/RM, and O/R mapping) in computer software is a programming technique for converting data between incompatible type systems in object-oriented programming languages.This creates, in effect, a “virtual object database” that can be used from within the programming language.”The concept is easy to say and difficult to imagine so we first of all try to draw it to have a better clarity.
To create a mapping, we require two ends so what are the two ends here?
One of the end is data and other is objects. Don’t you think, we should know both these terms first, before creating a mapping between the two ends.
Data is maintained in databases and these databases usually follows relational model like- we can have different tables in our database and these tables can be referenced by each other by following one of the relationships-
- One-to-One
- One-to-Many
- Many-to-One
- many-to-many
User.Name where User is an object and Name is the state of the object.
Now, if we look from the developer’s point of view, then it is
clearly visible that how easily we can operate an object. Just imagine,
in similar way,if we can operate on our data then our life will become
much more comfortable.To achieve that, we need to create a mapper between our data and objects like-
Here,Mapper does a transition between those database and objects. For example, it transits all related records from a table to the mapped object . Mapper is the layer of abstraction that hides all the implementation details, we don’t handle all the sql overhead but just work with the generated useful Objects,the OOP way.
What is the need of Object-relational mapping? or What are the benefits of Object-relational mapping?
- Simple: We have already discussed, how easy it is, to work with objects than a relational database.
- Less Code : Less code as compared to manual queries, handwritten stored procedures, triggers,functions
- Reduced development and maintenance costs: No big shots code required for regular database operations.
- Caching: Object caching provides improved system performance.
- Concurrency: Multiple users can do operations at the same time.
- Loading of related data: All objects which are related to the object called, are loaded simultaneously.
- Keys and Identifiers: Keys are important for the calls and they are properly managed.
- Transactions: Transactions Management rules are properly followed and any operation occurs within a transaction scope.
What are the options available?
NHibernate:NHibernate is a great ORM for the Microsoft .NET platform. NHibernate is free as open source software that is distributed under the GNU Lesser General Public License. Features:
- Mapping from .NET classes to database tables.
- Generates SQL commands.
- Query execution and data retrieval.
- Object oriented.
- Portable with little overhead.
LINQ to SQL is Microsoft’s answer to all the existing ORM’s available in the market and it has some nice features plus it is quite easy to get started with. It basically works with SQLClient adapters to communicate with database.
Features:
- Microsoft Support is the biggest plus point.
- Cannot be termed as full fledged ORM but captures the basic principle of ORM.
- LINQ is database independent and supports a very rich extensibility model which can be applied to any data source.
- Light-Weight.
- Easy Configuration.
Entity Framework is another entry from Microsoft which is very powerful.
The Entity Framework’s ORM implementation provides services like change tracking, identity resolution, lazy loading, and query translation so that developers can focus on their application-specific business logic rather than the data access fundamentals.
Features:
- Works with a variety of database servers (including Microsoft SQL Server, Oracle, and DB2)
- Includes a rich mapping engine that can handle real-world database schemas and works well with stored procedures
- Provides integrated Visual Studio tools to visually create entity models and to auto-generate models from an existing database. New databases can be deployed from a model, which can also be hand-edited for full control
- Provides a Code First experience to create entity models using code. Code First can map to an existing database or generate a database from the model.
- Integrates well into all the .NET application programming models including ASP.NET, Windows Presentation Foundation (WPF), Windows Communication Foundation (WCF), and WCF Data Services (formerly ADO.NET Data Services)
OpenAccess:
OpenAccess ORM is developed by the .NET solutions provider Telerik. There is no major difference in the functionality and features delivered by ORMs and OpenAccess.
Features:
- Visual Entity Designer with numerous wizards.
- LINQ support
- Disconnected Data support
- Caching support
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