This section describes the basic concepts of databases, including the role of database storage, the history of database technology, database storage structures and database development.
Data storage
Computer data (Data) is generally stored as data storage hard disk space resources, to ensure that the data in the computer can continue to save. For processing of data, typically using a database associated with the processing techniques to ensure efficient data processing.
Using a database management not only improves the efficiency of data storage, but also on the storage level to improve the security of the data. By storing pattern classification makes data management more safe and convenient, better implementation calls and comparison of data, and easy to use query operations.
What is a database
Database data management is an effective technique, an ordered set consisting of a group of data, these data are stored in a structured data table. Linkages between data tables, reflect the nature of the link between objective things. Database can effectively help an organization or enterprise scientific management of all types of information resources.
Data are basic objects stored in the database, is arranged in a certain order physical symbol combinations. There are many forms of data, it can be numbers, text, images and even audio or video, they can go through digitized into the computer.
Database is a collection of data, have a uniform structure and stored in the storage medium in a unified, integrated multiple data applications, and may be shared by each application.
In daily life, people can directly describe objective things Chinese, English and other natural languages. In the computer, we will have the characteristics of abstract things of interest and a recording composition will be described.
For example, student records, the student information by school, name, sex, age, place of birth, telephone number and other characteristics of the composition, then one of these specific characteristic values of the configuration of a recording is a student information data, such as "2016010102 Joe Smith, Male, 26, Shanxi, School of computer Science, 185 ********. "
It is worth noting that the description in the form of data can not fully express the content, you need to go through interpretation. For example, for above this student records, to understand its meaning people will get the message: John's student number is 2016010102, 26 years old this year, Shanxi, studied at the School of Computer Science, his contact telephone number is 185 **** ****; and do not understand the semantics of the people can not understand its meaning. Therefore, the data and interpretation of the data are inseparable, interpretation of the data refers to the description of the meaning of the data, meaning data is also known as semantic data, so that data and its semantics are inseparable, there is no semantic data is meaningless and incomplete .
Database storage structures
Database storage structure that describes the relationship between the database and the physical representation of data and logic data, physical data and logical data mapping method. In databases, the data can be used in two forms described objective reality: physical data and logical data described below. Conversion between physical data and logical data through the database management system implemented.
Physical Data Description
Refers to a physical description of the data is stored on the data storage device, physical data is actually stored on a data storage device, also called a physical data recording. The position of the physical records stored in the store can be divided into ordered and disordered storage.
In the physical description data, data describing the terms used include the following.
- Bit (bit): a unit is called a binary bit, bit 1 or 0 can only take.
- Byte (byte): 8 bits is called a byte can store a character corresponding to the ASCII code.
- Word (word): word composed of several bytes. A word contained in the binary bits called word length in many computer are different, for example, a computer word length may be 8, 16, 24, 32 and the like.
- Block (block): minimum memory and external memory units in the exchange of information, also called a physical block or physical records, the size of each block is typically 256 bytes, 512 bytes, 1024 bytes and so on.
- Vol (volume): a useful information all the input-output device can be loaded, for example, a tape drive is the volume, a disk pack disk device is a roll.
- Storage disorder (unordered): storing the data records in the order of insertion.
Logical Data Description
Logical data description refers to the user or programmer for the operation of the data format, the data logic is an abstract concept, the objective and the record to reflect the real world, these data may also be referred to as logical records.
Logical data consists of two levels, one level is a description of the objective reality of the world of information, another level is a description of the database management system data.
In the description of the objective reality of the world of information, the terms used include the following.
- Entity (entity): the existence of objective reality thing use to describe the entity. Entity may either be specific, tangible objects, may be abstract, invisible object. For example, a book is a tangible object library once the process is an invisible object.
- Set entity (entities): a set of identical characteristics similar entities called entity sets. For example, a library of all books is a set of entities, all of the library's borrowing process is a set of entities.
- Property (attribute): characteristic entities called attributes. Each attribute has a range, the range may be an integer type, float type, date type, or other types of characters. For example, properties include physical book title, ISBN, publication date, number of pages, the price and other publishers, these corresponding to the attribute range are character type, character type, date type, type integer, floating point numbers and character type Wait.
- Identifier (identifier): uniquely identify the attribute or set of attributes of each entity. For example, the book ISBN attribute is the identifier of a physical book, borrowing process entity identifier including library card number, ISBN two properties.
The logical data will ultimately be converted into a physical data through the database management system. In the database management system, data describing the logic of the terms, including what does?
Below relational database management system as an example are introduced.
- The data item (data item): also called a field (Field), marker entity properties can be named minimum unit, generally designated data item descriptive name of the attribute. These names may be Chinese, English or Pinyin.
- Tuple (tuple): also called a recording (record), a set of data items referred to as a tuple. A tuple represents a particular entity.
- Relation (relation): In the relational database system, a set of tuples in the same class where the called relationship. Relation is suitable entity set, which includes all the tuples of a set of entities. For example, all books can form a relationship books.
- Key code (key): In the relational database system, capable of uniquely identifying the relationship data item or combination of data items for each tuple of the relation is referred to as the key code.
After two logical entities described objective data, and finally into the actual physical data storage.