![]() ![]() Applications depend on the logical schema.Physical Data Independence – the ability to modify the physical schema without changing the logical schema Instance – the actual content of the database at a particular point in timeĤ. Logical schema: database design at the logical levelģ.Physical schema: database design at the physical level.Analogous to type information of a variable in a program.e.g., the database consists of information about a set of customers and accounts and the relationship between them).Schema – the logical structure of the database.Similar to types and variables in programming languages.Views can also hide information (e.g., salary) for security purposes. View level: application programs hide details of data types.Name : string street : string city : integer end Logical level: describes data stored in database, and the relationships among the data.Physical level describes how a record (e.g., customer) is stored.Database systems offer solutions to all the above problems two people reading a balance and updating it at the same timeĢ. Uncontrolled concurrent accesses can lead to inconsistencies transfer of funds from one account to another should either complete or not happen at allĬoncurrent accessed needed for performance Drawbacks of using file systems (cont.)įailures may leave database in an inconsistent state with partial updates carried outĮ.g.Hard to add new constraints or change existing ones #Jntu dbms notes ppt code#account balance > 0) become part of program code
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