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【正文】 of data types. Views can also hide information (., salary) for security purposes. 169。Silberschatz, Korth and Sudarshan Database System Concepts Instances and Schemas ? Similar to types and variables in programming languages ? Schema – the logical structure of the database ? ., the database consists of information about a set of customers and accounts and the relationship between them) ? Analogous to type information of a variable in a program ? Physical schema: database design at the physical level ? Logical schema: database design at the logical level ? Instance – the actual content of the database at a particular point in time ? Analogous to the value of a variable ? Physical Data Independence – the ability to modify the physical schema without changing the logical schema ? Applications depend on the logical schema ? In general, the interfaces between the various levels and ponents should be well defined so that changes in some parts do not seriously influence others. 169。Silberschatz, Korth and Sudarshan Database System Concepts EntityRelationship Model Example of schema in the entityrelationship model 169。Silberschatz, Korth and Sudarshan Database System Concepts Relational Model ? Example of tabular data in the relational model customer name Customerid customer street customer city account number Johnson Smith Johnson Jones Smith 192837465 019283746 192837465 321123123 019283746 Alma North Alma Main North Palo Alto Rye Palo Alto Harrison Rye A101 A215 A201 A217 A201 Attributes 169。Silberschatz, Korth and Sudarshan Database System Concepts Data Definition Language (DDL) ? Specification notation for defining the database schema ? . create table account ( accountnumber char(10), balance
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