SQL is based on relational algebra and tuple relational calculus, including data definition language and data operation language. The scope of SQL includes data insertion, query, update and deletion, database schema creation and modification, and data access control. Although it is largely declarative programming (4GL), it also contains elements of procedural programming.
SQL is the first commercial language implementation of edgar codd's relational model, which is described in 1970 in his influential paper Relational Model of Large * * * Databases.
Although SQL is not completely designed according to the relational model of Cod, it is still the most widely used database language. SQL became the standard of American National Standards Institute (ANSI) in 1986 and the standard of International Organization for Standardization (ISO) in 1987. Since then, this standard has undergone a series of updates and added many new features.
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SQL is an advanced non-procedural programming language, which allows users to work on advanced data structures. It does not require users to specify the storage mode of data, nor does it require users to know the specific storage mode of data. And its interface enables database systems with completely different underlying structures and different databases to use the same SQL as data input and management.
It takes a set of records as the manipulated object, and all SQL statements accept this itemset as input and return the submitted itemset as output. This itemset feature allows the output of one SQL statement as the input of another, so SQL statements can be nested, which makes it very flexible and powerful.
In most cases, in other programming languages, it takes a long program to practice an event, which can be expressed in only one sentence on SQL. This also means that you can write very complex statements in SQL without paying special attention to performance.
Baidu Encyclopedia-Structured Query Language