The four characteristics of database transactions are: atomicity, consistency, persistence, and isolation.
Atomicity: A transaction is the logical unit of work of the database. All modifications to the database are either executed or not executed at all. Consistency: Before and after a transaction, the state of the database satisfies all integrity constraints. Isolation: Concurrently executed transactions are isolated, and one does not affect the other. If there are two transactions, running at the same time, performing the same function, there will only be one request for the same data at the same time. Setting the isolation level of the database can achieve different isolation effects. Durability: After a transaction is completed, the changes made to the database by the transaction are persisted in the database and will not be rolled back.
Data models are divided into two categories: conceptual models and logical models.
Conceptual model: Models data and information from the user's perspective, mainly used in database design.
Logical model: mainly includes hierarchical model, network model and relational model. It models data from the perspective of computer system and is mainly used for the implementation of DBMS.
From the perspective of database application developers, database systems usually adopt a three-level schema structure, which is the system structure within the database system. From the perspective of database end users, the structure of the database system is divided into: single-user structure, master-slave structure, distributed structure, client-server, browser-application server/database server multi-layer structure, etc.
What is big data?
Big data refers to huge data sets collected from many sources in multiple forms, often in real time. In the case of business-to-business sales, this data may come from social networks, e-commerce sites, customer visit records, and many other sources. These data are not normal data sets in the company's customer relationship management database.