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What is a database database is a collection of data stored in some organized way. To put it simply, a database is a warehouse that stores data all the time, which is organized and stored according to a certain data structure (data structure refers to the organizational form of data or the relationship between data). We can manage the data in the database (the database in the database) through a variety of methods provided by the database.

When we collect a large amount of data, we should save it for further processing and extract the information we need.

Database ranking Network: https://db-engines.com/en/ranking

About the types of NoSQL: http://www.nosql-databases.org/ databases

Databases are usually divided into three types: hierarchical database, network database and relational database. Different databases are linked and organized according to different data structures. Nowadays, there are two most commonly used database models in the Internet, namely, relational database and non-relational database.

Data types: structured data, semi-structured data, unstructured data. Structured data: structured data refers to data that can be represented and stored in a relational database in two-dimensional form. The general characteristics are: the data is a behavior unit, a row of data represents the information of an entity, and the attributes of each row of data are the same. To give an example: student number name age S001 322S002 Li Si 19S003 Wang Wu 33 semi-structured data: semi-structured data is a form of structured data, it does not conform to the data model structure associated with the form of relational databases or other data tables, but contains related tags to separate semantic elements and to layer records and fields. Therefore, it is also called self-describing structure. Semi-structured data, belonging to the same category of entities can have different attributes, even if they are grouped together, the order of these attributes is not important. Common semi-structured data are XML and JSON. For two XML files, the first one may be: A13female

The second possibility is:

Bmale from the above example, the order of attributes is not important, and the number of attributes in different semi-structured data is not necessarily the same. Some people say that semi-structured data is data stored in the data structure of trees or graphs, how to understand it? In the above example, the label is the root node of the tree, and the tag is the child node. Through this data format, you can freely express a lot of useful information, including self-description information (metadata). Therefore, the scalability of semi-structured data is very good. Unstructured data:

As the name implies, there is no fixed structure of data. All kinds of documents, pictures, video / audio and so on belong to unstructured data. For this kind of data, we generally store it directly as a whole, and generally store it in a binary data format.

1. The model of relational database is to transform complex data structure into simple binary relationship (two-dimensional table form). In the relational database, the operation of the data is almost based on one or more relational tables, and the data management is realized by classifying, merging, connecting or selecting these related tables.

Introduction of typical products of relational database: Oracle, Mysql and their variants, PostgreSQl and so on.

An example of the relationship between tables in Relational Database

Student student number name age S001 Zhang 322S002 Li Si 19S003 Wang Wu 33 course number course name credit C001Linux25C002Python25C003Mysql25C004Java25 course number course number score S001C00188S001C00192S001C00178S001C00184

2. Introduction of non-relational database

Non-relational database generally refers to NoSQL,NoSQL (NoSQL = Not Only SQL). With the rise of the Internet Web2.0 website, the traditional relational database has been unable to cope with the Web2.0 website, especially the super-large-scale and highly concurrent SNS type Web2.0 pure dynamic website, which has exposed many insurmountable problems. NoSQL is a new data revolutionary movement, and it has been suggested since early morning that the trend is getting higher and higher in 2009. Non-relational time library has developed rapidly because of its own characteristics. NoSQL database can play an unimaginable high efficiency and high performance in specific scenarios, which is an effective supplement to the traditional relational database. Key-value storage database (Key-Value)

A key-value database is similar to a hash table used in traditional languages, which has a specific key and a pointer to specific data. Databases can be added, queried, and deleted through key, because key primary keys are used to access data, resulting in high performance and scalability. The advantages of Key-Value model for IT system are simple, easy to deploy and high concurrency. Typical products: Redis, Memcached

3. Column storage database (Column-oriendted)

Column storage databases store data in column families, which store relevant data that is often queried together, such as humans, and we often query someone's name and age rather than home address. In this case, the name and age are placed in one column family, and the home address is placed in another column family. This kind of database is usually used for clothing distributed storage of huge amounts of data. Typical product: HBase

4. Document oriented database (Document-Oriented)

The document-oriented database stores the data as text. Each document is a self-contained data unit, a collection of a series of data items, each with a name and a corresponding value, which can be simple data types, such as strings, numbers, dates, etc., or complex types, such as ordered lists and associated objects. The smallest unit of data storage is the document, the document properties stored in the same table can be different, and the data can be stored in many forms such as XML, JSON, or JSONB. Typical products: MongoDB, CouchDB

5. Graphic database

The graphic database allows us to store the data in a graphical way. Entities are treated as vertices, while relationships between entities are used as edges. For example, if we have three entities, Steve Jobs, Apple, and Next, we will have two "Founded by" edges that connect Apple and Next to Steve Jobs. Typical products: Neo4J, InforGrid

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