CISC 332 Lecture Notes - Lecture 3: Data Manipulation Language, Data Definition Language, Edgar F. Codd
10 Feb 2018
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Document Summary
Exam q: why should you store data in a flat file. External views allow data to be customized to users or groups of users. Logical data described in terms of the dbms model. Physical storage info such as file organization. Data independence application programs are insulated from changes in the way the data is structured and stored. Logical data independence if logical structure changes, views can be adjusted to maintain the structure originally seen by the user. Physical data independence if the physical structure changes, the logical level maintains the structure of the data. Schema the logical structure of the db o. Physical schema vs logical schema exactly how it sounds. Instance the actual content of the db at a particular time. Entity relationship model a picture representation of the schema o. Relational model looks like the actual tables and data. Data definition language (ddl) used to define schemas. Data manipulation language (dml) used to access and manipulate data.
