Data Dictionary

In a relational database, the metadata in the data dictionary includes the following:

• Names of all tables in the database and their owners
• Names of all indexes and the columns to which the tables in those indexes relate
• Constraints defined on tables, including primary keys, foreign-key relationships to other tables, and not-null constraints
  

For most relational database management systems (RDBMS), the database management system software needs the data dictionary to access the data within a database. For example, the Oracle DB software has to read and write to an Oracle DB. However, it can only do this via the data dictionary created for that particular database.

For instance, suppose that in a commercial bank's database, the administrator wants to determine which table holds information about loans. Making an educated guess that the table most likely has the word "LOAN" in it, he would issue the following query on the data dictionary (the first query is for an Oracle DB, while the second is for an SQL Server DB):

• SELECT * FROM DBA_TABLES WHERE TABLE_NAME LIKE '%LOAN%';
• SELECT * FROM SYSOBJECTS WHERE TYPE='U' AND NAME LIKE '%LOAN%';