SQL是高级的非过程化编程语言，是沟通数据库服务器和客户端的重要工具，允许用户在高层数据结构上工作。它不要求用户指定对数据的存放方法，也不需要用户了解具体的数据存放方式，所以，具有完全不同底层结构的不同数据库系统，可以使用相同的SQL语言作为数据输入与管理的接口。它以记录集合作为操作对象，所有SQL语句接受集合作为输入，返回集合作为输出，这种集合特性允许一条SQL语句的输出作为另一条SQL语句的输入，所以SQL语句可以嵌套，这使它具有极大的灵活性和强大的功能，在多数情况下，在其他语言中需要一大段程序实现的功能只需要一个SQL语句就可以达到目的，这也意味着用SQL语言可以写出非常复杂的语句。-SQL is advanced the process programming language, is communication database server and client important tool that allows users to work on top in data structure. It does not require the user to the storage of data specified method, also do not need to users understand the specific data storage way, so, with a fully different ground floor of the different structure database system, can use the same SQL language input and management as data interface. It set a record as the operation object, all the SQL statement accept set as input, return to set as the output, this set features allow an SQL statement output as another an SQL statement input, so the SQL statement may be nested, this make it has great flexibility and powerful functions, in most cases, in other languages need a large section of the program of the realization of the function only need a SQL statement be able to achieve a goal, it also means using SQL language can write very complex statement.