SQL language is not a standard language for hierarchical databases, but a standard language for relational databases. Structured Query Language SQL is a comprehensive, versatile, and highly functional relational database language; used for accessing data and querying, updating, and managing relational database systems.
SQL (Structured Query Language, structured query language) is a special purpose programming language, a database query and programming language, Used to access data and query, update and manage relational database systems.
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Structured Query Language is an advanced non-procedural programming language that allows users to work on high-level data structures. It does not require users to specify the data storage method, nor does it require users to understand the specific data storage method, so different database systems with completely different underlying structures can use the same structured query language as the interface for data input and management. Structured Query Language statements can be nested, which makes it extremely flexible and powerful.
SQL can be functionally divided into three parts: data definition, data manipulation and data control.
The core part of SQL is equivalent to relational algebra, but it has many features that relational algebra does not have, such as aggregation, database updates, etc. It is a comprehensive, universal, and extremely powerful relational database language. Its characteristics are:
1. Integration of data description, manipulation, control and other functions.
2. Two usage methods, unified grammatical structure. SQL can be used in two ways. One is online interactive use. SQL in this way is actually used as a self-contained language. Another way is to embed it into some high-level programming language (such as C language, etc.) for use. The former method is suitable for non-computer professionals, and the latter method is suitable for professional computer personnel. Although they are used in different ways, the grammatical structure of the languages used is basically the same.
3. Highly non-procedural. SQL is a fourth-generation language (4GL). Users only need to ask "what to do" and do not need to specify "how to do it". Access path selection and specific processing operations are automatically completed by the system.
4. The language is concise, easy to learn and use. Although SQL has powerful functions, the language is very concise, and only 9 verbs are used for the core functions. The syntax of SQL is close to spoken English, so it is easy for users to learn and use.
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