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3 Oracle Data DictionaryThe Oracle data dictionary is one of the most important components ofthe Oracle DBMS.It contains all information about the structures and objects of thedatabase such as tables,columns, users, data files etc. The data stored in the data dictionary arealso often calledmetadata. Althouh it is usually the domain of database administrators!DBAs", the datadictionary is a #aluable source of information for end users andde#elopers. The data dictionaryconsists of t$o le#els% the internal le#el contains all base tables that areused by the #ariousDBMS soft$are components and they are normally not accessible by endusers. The e&ternal

le#el pro#ides numerous #ie$s on these base tables to access informationabout objects andstructures at different le#els of detail.

3.' Data Dictionary TablesAn installation of an Oracle database al$ays includes the creation of threestandard Oracleusers%( S)S% This is the o$ner of all data dictionary tables and #ie$s. This userhas the hihestpri#ilees to manae objects and structures of an Oracle database such ascreatin ne$users.( S)ST*M% is the o$ner of tables used by different tools such S+-orms,S+-/eports etc.This user has less pri#ilees than S)S.( 01BI2% This is a dummy4 user in an Oracle database. All pri#ileesassined to thisuser are automatically assined to all users 5no$n in the database.The tables and #ie$s pro#ided by the data dictionary contain informationabout( users and their pri#ilees,

( tables, table columns and their data types, interity constraints,inde&es,( statistics about tables and inde&es used by the optimi6er,( pri#ilees ranted on database objects,( storae structures of the database.The S+ commandselect from DI2T7IO8A/)9:lists all tables and #ie$s of the data dictionary that are accessible to theuser. The selectedinformation includes the name and a short description of each table and#ie$. Before issuin

this ;uery, chec5 the column definitions of DI2T7IO8A/)9 usin descDI2T7IO8A/)9 and set

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the appropriate #alues for column usin the format command.The ;ueryselect from TAB:retrie#es the names of all tables o$ned by the user $ho issues thiscommand. The ;uery

select from 2O:

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( A % /o$s in the A #ie$s include ro$s of the 1S*/ #ie$s and allinformation aboutobjects that are accessible to the current user. The structure of these#ie$s is analoousto the structure of the 1S*/ #ie$s.