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This picture is from ANSI/SPARC (Standards Planning And Requirements Committee) for a layered model of database architecture:
- External level is the one closest to the users—i.e., it is the one concerned with the way the data is viewed by individual users.
- Conceptual level is a “level of indirection” between the other two.
- Internal level is the one closest to physical storage—i.e., it is the one concerned with the way the data is physically stored.
- Schema is the description of the organization of a database as seen from the three levels.
- Data definition language (DDL) allows a database administrator to define the database structure, schema, and subschema components.
- Data manipulation language (DML) is the language (set of commands) used to manipulate the data in the database.
- Data administration is for the planning, organization, control, and monitoring of the entire data resource.
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A man is struggling to find a parking space. “Lord,” he prays. “I can’t stand this. If you open a space up for me, I swear I’ll give up the drink and go to mass every Sunday.” Suddenly, the clouds part and the sun shines on an empty parking spot. Without hesitation, the man says: “Never mind, I found one!” |