What are the relational databases?
What are the relational databases?
What do you need so many for? Most databases nowadays are relational databases ah. Oracle, SQLServer, Sybase, Informix, access, DB2, mysql, vfp, NPC gold warehouse (domestic, I’ve used) as long as you think you can, what xml can be used as a relational database ah. Exactly 10. I hope my answer is helpful to you!
What are the commonly used relational databases
Relational model refers to the two-dimensional table model, and thus a relational database is a two-dimensional table and its linkage between the composition of a data organization. The current mainstream relational database Oracle, DB2, MicrosoftSQLServe radical, Microsoft Access, MySQL and so on.
What are the relational databases
Large ones are:
Oracle, SQL Server, DB2, Mix, Sybase, etc.
Open-source ones are:
MySQL, Postpresql, etc.
< p>File-based ones are:
Access, SQLAnywhere, sqlite, interbase
What is a relational database
Introduction to relational databases Relational databases store data in the form of rows and columns to make it easier for the user to understand. This series of rows and columns is known as a table and a set of tables make up a database. Users use queries (Query) to retrieve data from the database. A Query is a SELECT statement used to specify rows and columns in a database. A relational database usually contains the following components: Client application (Client) Database server (Server) Database (Database) StructuredQueryLanguage (SQL) Client and Server side of the bridge, the Client with SQL to send requests to the Server side, the Server returns the Client’s request. Server returns the results requested by the Client. Now popular large-scale relational database IBMDB2, IBMUDB, Oracle, SQLServer, SyBase, Informix and so on. Relational databases are not the only advanced database model, and not at all the model with the best performance, but relational databases are indeed the most widely used and easiest to understand and use database model today. Most enterprise-level system databases use relational databases, and the concept of relational databases is fundamental to mastering database development, so the questions in this section have become one of the frequent questions in .NET interviews. Knowledge Points Involved Concepts of Relational Databases Benefits of Relational Databases Analyzing the Problems Concepts of Relational Databases The so-called relational database refers to a database that employs a relational model to organize data. The relational model was first proposed by Dr. E.F. Codd, a researcher at IBM, in 1970, and in the decades that followed, the concept of the relational model was fully developed and gradually became the dominant model of database architecture. Simply put, the relational model refers to a two-dimensional table model, and a relational database is a data organization composed of two-dimensional tables and the links between them. The following is a list of commonly used concepts in the relational model. Relationship: can be understood as a two-dimensional table, each relationship has a relationship name, which is commonly referred to as the table name. Tuple: can be understood as a row in a two-dimensional table, often referred to as a record in databases. Attribute: can be understood as a column in a two-dimensional table, often referred to as a field in the database. Field: the range of values of an attribute, that is, the limit of values of a column in the database. Keyword: a set of attributes that uniquely identifies a tuple. Often referred to as a primary key in databases, it consists of one or more columns. Relationship Schema: A description of a relationship in the format: Relationship Name (Attribute 1, Attribute 2, …, Attribute N). It is often referred to as a table structure in databases. Advantages of Relational Databases Relational databases have the following advantages over other models of databases: Ease of Understanding: The two-dimensional table structure is a concept that is very close to the logical world, and the relational model is easier to understand compared to other models such as mesh and hierarchy. Easy to use: the common SQL language makes it very easy to operate relational databases, programmers and even data administrators can easily operate the database at the logical level without having to understand the underlying implementation. Easy to maintain: rich integrity (entity integrity, referential integrity and user-defined integrity) greatly reduces the probability of data redundancy and data inconsistency. In recent years, non-relational databases have developed rapidly in theory, such as: mesh model, object model, semi-structured model and so on. The mesh model has the advantage of higher performance, usually applied in systems with high performance requirements; object model conforms to the idea of object-oriented applications, which can be perfectly articulated with the program without the need for another intermediate conversion components, such as many of the current O\RMapping components; semi-structured model with the development of XML and the development of the wow now there are a lot of semi-structured database model. However, by virtue of its theoretical maturity, ease of use and the wide range of existing applications, relational databases are still the mainstream of system applications.
What are the non-relational databases
NoSQL, which is now popular,
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What is a relational database?
A relational database is a database that uses a relational model to organize data. The relational model was first proposed by Dr. E.F. Codd, a researcher at IBM, in 1970. In the following decades, the concept of the relational model has been fully developed and has gradually become the dominant model of database architecture. Simply put, the relational model refers to a two-dimensional table model, and a relational database is a data organization composed of two-dimensional tables and the links between them. The following is a list of commonly used concepts in the relational model. Relationship: can be understood as a two-dimensional table, each relationship has a relationship name, which is commonly referred to as the table name. Tuple: can be understood as a row in a two-dimensional table, often referred to as a record in databases. Attribute: can be understood as a column in a two-dimensional table, often referred to as a field in the database. Field: the range of values of an attribute, that is, the limit of values of a column in the database. Keyword: a set of attributes that uniquely identifies a tuple. Often referred to as a primary key in databases, it consists of one or more columns. Relationship Schema: A description of a relationship in the format: relationship name (attribute 1, attribute 2, …, attribute N). Often referred to as a table structure in databases.
What kind of database information is available in relational databases
Lots of it. The relational ones are SQLServer, Sybase, Informix
mysql. and so on.
Real-time I know: LotusNotes. Including XML which can also be used as a real-time database.
What is a relational database?
Relational databases store data in the form of rows and columns so that it is easy for users to understand. This series of rows and columns is known as a table and a set of tables make up a database. Tables are related to each other with data records. The user uses a query (Query) to retrieve data from the database. A Query is a SELECT statement used to specify rows and columns in a database. A relational database usually contains the following components: Client application (Client) Database server (Server) StructuredQueryLanguage (SQL) A bridge between the Client and Server, where the Client uses SQL to send requests to the Server, and the Server returns the results requested by the Client. The Server returns the results requested by the Client. Now popular large-scale relational database IBMDB2, Oracle, SQLServer, SyBase, Informix, access, foxpro and so on.
What are the relational databases?
The current mainstream relational databases are Oracle, DB2, MicrosoftSQLServer, Mic nailosoft
Access, MySQL, etc.
What are the challenges faced by the traditional relational databases?
Challenge 1: Data sources are intricate
An abundant data source is a prerequisite for the development of big data industry. The total amount of digitized data resources in China is far lower than that of the United States and Europe, with the annual amount of new data being only 7% of that of the United States and 12% of that of Europe, of which the accumulation of data resources in the *** and manufacturing industries lags far behind that of foreign countries. As far as the existing limited data resources are concerned, there is also standardization, low accuracy, low completeness and low utilization value, which greatly reduces the value of the data
Challenge 2: Data Mining and Analysis Model Establishment
Being stepped into the era of big data, people are talking about big data, and it seems that this has evolved into a new trend trend. More than ever, data is rooted in every corner of our lives. We are trying to use data to solve problems, improve welfare, and enable new economic prosperity
Challenge 3: The trade-off between data openness and privacy
The premise of data application is data openness, which is already a consensus. Some professionals have pointed out that China’s population is the largest in the world, but in 2010 China’s newly stored data was 250 petabytes, which is only 60 percent of Japan’s and North America’s 7 percent. At present, some departments and organizations in China have a large amount of data but prefer not to use it themselves rather than provide it to the relevant departments for sharing, resulting in incomplete information or duplication of investment.In 2012, China’s data storage reached 64EB, of which 55% of the data requires a certain degree of protection, however, at present less than half of the data is protected!
What are the relational databases?
Currently the mainstream of large databases, medium-sized databases, as well as personal and small databases are almost all relational databases, such as ORACLE, SQLSERVER, MySQL, SyBase, Access and so on.
What kinds of databases are there?
Commonly used databases: oracle, sqlserver, mysql, access, sybase2, features. -oracle: 1. Database security is very high, very suitable for large databases. Supports a variety of system platforms (HPUX, SUNOS, OSF/1, VMS, WINDOWS, WINDOWS/NT, OS/2).2. Supports client/server architecture and mixed architecture (centralized, distributed, client/server). -sqlserver:1. true client/server architecture. 2. graphical user interface, so that the system management and database management is more intuitive, simple. 3. has a very good scalability, can be used across a variety of platforms from running Windows95/98 laptop to running Windows2000 large multi-processor. -mysql:MySQL is an open source small relational database management system developed by MySQLAB of Sweden. 92HeZu.com gives free MySQL. currently MySQL is widely used in small and medium-sized websites on the Internet. MySQL is widely used in small and medium-sized websites on the Internet. Due to its small size, high speed, low total cost of ownership, and especially the open source feature, many small and medium-sized websites have chosen MySQL as their website database in order to reduce the total cost of ownership. -accessAccess is a desktop database that is only suitable for applications with a small amount of data, and it is good and efficient when dealing with a small amount of data and a single-access database. But it cannot have more than 4 simultaneous access clients. –
What are the types of database relational schema?
In a relational database there are two type structures, type and value. The relational schema is the type and the relationship is the value; the relational schema is a description of the relationship.
Describing a relationship requires defining it in two ways: first, a relationship is essentially a two-dimensional table, with each row of the table being a tuple and each column being an attribute. A tuple is an element of the Cartesian product of the set of attributes involved in the relation. A relationship is a collection of tuples, so the relationship schema must indicate the structure of this collection of tuples, i.e., what attributes it consists of, which domains these attributes come from, and the mapping relationship between the attributes and the domains.
The second aspect is that a relation is usually defined by the tuple semantics assigned to it. A tuple semantics is essentially an n-order predicate (n is the number of attributes in the attribute set). The totality of the elements in the Cartesian product that make that n-measure predicate true (or whatever portion of the elements conforms to the tuple semantics) constitutes the relation of that relational schema.
1.3.1 Basic concepts of relational database relational data, relational schema involves a number of concepts, terminology, beginners in this area is not easy to grasp and understand, the following in layman’s terms to these concepts and terminology for a simple introduction.
1. Relationships Relationships (Relation) refers to the information of the entity in the database, that is, the data in the two-dimensional tables in the database. A relationship is a database table values, the contents of the table is corresponding to the value of the relationship schema at a certain moment is called a relationship. For example, relationship A represents all the data recorded in a database that has a data table with the name A. Each relation in a relational database has the following six properties: ((1) The columns are homogeneous. That is, the components in each column are of the same type of data, from the same domain.
(2) Different columns can come from the same domain, saying that each column in it is an attribute, and different attributes are given different attribute names.
(3) The order of the columns does not matter. That is, the order of the columns can be exchanged arbitrarily.
(4) Any two tuples cannot be identical.
(5) The order of rows does not matter. That is, the order of rows can be swapped arbitrarily.
(6) The components must take on atomic values. That is, each component must be an indivisible database attribute.
2. Schema Schema (Schema) is a description of the logical structure and characteristics of all the data in the database, is a public view of the data for all users, also known as the logical schema. There are the following aspects of nature: ((1) a database has only one schema.
(2) The schema is a view of the data at the logical level.
(3) It is based on a particular data model.
Defining a schema involves not only defining the logical structure of the data, including the composition of the data items, their names, types, value ranges, etc., but also defining the security and integrity requirements associated with the data, and defining the linkages between these data.
3. RelationSchema The RelationSchema describes the table structure of a two-dimensional table corresponding to a relationship, i.e., what attributes are contained in the relationship, what domains the attributes come from, and the mapping relationship with the domains.
Difference between RelationSchema and Relationship: ((1) RelationSchema describes the relational data structure and semantics, and is the type of relationship. Whereas a relation is a collection of data, a value of the relational schema, an instance of the relational schema.
(2) A relation is actually the state or content of the relational schema at a given moment. A relational schema is static and stable, while a relationship is dynamic and constantly changing over time because database operations constantly update the data in the database.
4. Tuple Tuple (Tuple) is a basic concept in relational databases, each row in a relational table is a tuple. That is to say, each record in the database table is a tuple, each column of the table structure is an attribute, in two-dimensional tables, tuples are also known as records. A tuple can represent a relationship or a link between relationships.
In general, each record in a relational data table has a unique number (record number), which is also called a tuple number.
5. Code code (Key) is a basic concept in relational database systems. A code is a set of attributes that uniquely identifies an entity, the entire set of attributes, rather than a single attribute. In relational databases, codes include various types, such as hypercodes, candidate codes, and master codes.
((1) Supercode (SuperKey). A supercode is a collection of one or more attributes that uniquely identify an entity within an entity set. If K is a supercode, then any superset of K is also a supercode, i.e. if K is a supercode, then all sets containing K are also supercodes. For example, if a student is an entity, then the set of students is a set of entities, and hypercodes are used to distinguish between different students in the set of students. Suppose the student (entity) has several attributes: school number, ID number, name, and gender. Since a unique student can be found by the student number, {student number} is a hypercode, and similarly {student number, ID number}, {student number, ID number, name}, {student number, ID number, name, gender}, {ID number}, {ID number, name}, {ID number, name, gender} are also hypercodes. Here, since different students may have the same name, the name does not distinguish a student, i.e., {Name} is not a supercode, nor is {Gender}, {Name, Gender}.
(2) CandidateKey. A CandidateKey is the minimum set of attributes that can uniquely identify a tuple. CandidateKeys are selected from hypercodes, so a CandidateKey is also a collection of one or more attributes. Because supercodes are so broad and many are useless, candidate codes are minimal supercodes, and no true subset of them can be a supercode. For example, if K is a supercode, then all sets containing K cannot be candidate codes; if neither K, nor J is a supercode, then it is possible that the set {K, J} consisting of K and J is a candidate code.
While supercodes can uniquely identify an entity, it is likely that most supercodes contain redundant attributes, so candidate codes are needed.
For example, in the student table, student (student number, name, age, gender, major), the student number uniquely identifies a tuple, so the student number can be a candidate code. Since the student number can be used as a candidate code, the combination of the attributes student number and name can uniquely distinguish a tuple. At this point, the school number can be a code, and the combination of school number and name can be a code, but the combination of school number and name cannot be a candidate code, because even if the name attribute is removed, the remaining school number attribute is perfectly capable of uniquely identifying a tuple. That is, all of the attributes in a candidate code are required, and the absence of any one of them does not uniquely identify a tuple.
(3) PrimaryKey. PrimaryKey is any one of several candidate codes selected as the primary key, and this selected candidate code is called the primary code. If there is only one candidate code, then the candidate code is the primary code. Although the choice of the master code is relatively arbitrary, but in the actual development of a certain amount of experience is required, otherwise the development of the system will have problems. Generally speaking, the master code should be chosen for attributes that never or rarely change.
For example, in an employee entity, Employee (Employee Number, Name, Entry Time, Department, Position, Salary, Grade, Seniority, Phone), Employee Number can be used to uniquely identify a tuple in the entity, so Employee Number is a candidate code. If the combination of the entity attributes – name, entry time, department – can also uniquely identify a tuple, then (name, entry time, department) is also a candidate code. Either of these two candidate codes can be used as the master code for the employee entity, and it is generally easiest and most convenient to choose the employee number directly as the master code for the entity.
1.3.2 Definition of Relational SchemaRelationships are data records in a two-dimensional table of a database, and the relational schema is the table structure of a two-dimensional table of a database; relationships are dynamic, and the relational schema is static.
A relational schema can be described by six elements, R, U, D, dom, I, and F. Where R is the name of the relation;
U is the set of attribute names that make up the relation; D is the set of domains of the attributes in the set of U; dom is the mapping of the set of attributes, U, to the set of domains, D; I is the set of completeness constraints; and F is the set of dependencies on the data between attributes.
A relational schema is usually denoted as R(U, D, dom, I, F), or other elements can be ignored and directly simplified as R(U) or R(A1, A2, A3,…, An), where A1, A2, A3,…, An are attribute names.
For example, in a course selection module, there are relationship entities such as “Student”, “Course”, “Elective”. “The attributes of the “Student” entity are SNO (student number), SNAME (name), AGE (age), SEX (gender), SDEPT (department), of which “student number” is the primary key. The attributes of the “course” entity are CNO (course number), CNAME (course name), CDEPT (department), TNAME (instructor), where “course number” is the primary key; the attributes of the “elective” entity are GRADE (grade), SNO (student number), CNO (course number), of which “student number” and “course number” are joint primary keys. There is a many-to-many relationship between students and courses, i.e. a student can take more than one course at the same time, and a course can be taken by more than one student at the same time. This many-to-many relationship can be transformed into two one-to-many entities by using the “Elective” relationship entity as an intermediate bridging entity, as shown in the figure.
Diagram Student Election Entity
From the Entity Relationship Diagram of the diagram, we can get the set of Entity Relationship Patterns of the Election Module — Student Relationship, Course Relationship, Election Relationship. relationship schemas are as follows: student relationship schema Student (SNO, SNAME, AGE, SEX, SDEPT);
course relationship schema Course (CNO, CNAME, CDEPT, TNAME);
elective relationship schema StudentCourse (SNO, CNO, GRADE) .
Instantiation of the three relationship schemas defined above, after inserting the initialization data, you can get the instances of the three relationships of student, course, and elective, as shown in the figure. The rectangular box circled part of the figure is the relationship schema (table structure) in the course selection module; the elliptical box circled part of the relationship (data) in the course selection module. The table environment of the entire course selection module consists of two parts: the relationship schema and the relationship, one without the other. The decomposition of the relational schema standard relational schema normalization process is actually a relational schema “decomposition” process, that is, logically independent information in a separate relational schema. Decomposition is the main method of solving data redundancy, but also a principle of normalization – the relational schema has a redundancy problem should be decomposed.
Database designers should refer to the theory of schema normalization when designing relational databases to keep the database schema as high as possible. Generally try to design relational databases into the Bass-Code paradigm (BCNF) pattern set, if the design into the Bass-Code paradigm (BCNF) pattern set can not meet the criteria for maintaining functional dependencies, then only to reduce the requirements, designed to be the third paradigm (3NF) pattern set, in order to achieve the function to maintain dependence and lossless decomposition of the basic requirements.
An example of the relationship between student, course, and elective
1. Definition of decompositionA relational schema can be decomposed into numerous sub-relational schemas, and the different ways of decomposition result in different sub-relational schemas.
Decomposition of a relational schema is the decomposition of a relational schema in a certain way to get all the sub-relational schemas.
For example, if a relation schema R is decomposed in a certain way, a relation set ρ={R1, R2, …, Rn} can be obtained. where the set of attributes U = U1 ∪ U2 ∪ … ∪ Un and there cannot exist Ui ⊆ Uj, 1 ≤ i, j ≤ n.
The set of functional dependency relations F = F1 ∪ F2 ∪ … ∪ Fn, where F1, F2, …, Fn are the relations of F on U1, U2, … …, Un the projections on Un.
2. The standard decomposition method for decomposing a low-level relational schema into a high-level relational schema is not unique, as long as it can ensure that the decomposed relational schema is equivalent to the original relational schema, it is a complete and standard decomposition method. The standard decomposition method of a relational schema should meet the following two requirements at the same time: ((1) The decomposition has lossless connectivity.
(2) The decomposition should maintain functional dependency.
Decomposition with lossless connectivity guarantees that information will not be lost, but lossless connectivity does not necessarily solve the problems of insertion anomalies, deletion anomalies, modification complexity, data redundancy, etc., and if these problems are to be solved, higher principles of Relational Data Paradigm Theory should be considered.
What are the types of databases?
There are two types of databases, relational and non-relational.
Databases, in short, can be regarded as electronic filing cabinets – places where electronic files are stored and users can add, intercept, update, delete and other operations on the data in the files.
Relational databases include:
Oracle, DB2, MicrosoftSQLServer, Microsoft Access, MySQL and so on.
Non-relational databases include:
NoSql, Cloudant, MongoDb, redis, HBase and so on.
Expanded information: