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视图是从一个或几个基本表或视图导出的表,视图中的数据依旧存放在基本表中,因此视图成为虚表。 1、创建视图 CREATE VIEW S_AVG(classno,S_avg) AS Select classno,AVG(point) from ...
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DML(Data Manipulation Language)数据操纵语言。 插入、删除、更新 1、插入元组 (1)一次插入一条数据 insert into Student values('0922278926','李翔','男','199...
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1、等值连接与非等值连接 当连接运算符为等号时为等值连接,当为其他运算符时为非等值连接。 select studentno,sname from student,teach_class,teacher where student.class...
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(1)关键字and or > < = >= <= between ….and not between…and select sname from Student where studentno='2017...
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DDL(数据库定义语言) 对数据库、表、列(属性)的操作。 1、创建表 -创建表时可以添加完整性约束 create table mytable --创建表 ( id int not null, name nchar(8) null, sex...