ABSTRACT
The word database is used informally to refer to any collection of data. For example, one might call a file of e-mail addresses a database. There is a more formal definition of a database in an RDBMS. Information in an RDBMS is structured in a sort of hierarchy. An RDBMS contains databases, which contain schemas, which contain tables, which contain rows, which contain columns. A schema is a collection of tables. Rather than have tens or hundreds of tables in your database, the tables can be organized into schemas for clarity and convenience. In one sense, a schema is just a type of name space that allows a richer naming convention. For example, a database might contain several tables of structures. It might be convenient to use the table name structure to contain these structures. Rather than try to fit all the various structures together into a single table, or use coded table names such as nci_structures, pubchem_structures, or vendor_structures, separate schemas could be created. The fully qualified table names would then be nci.structure, pubchem.structure, and vendor. structure. The tables are segregated from each other by virtue of belonging to a separate schema, yet they are shown to be similar by sharing the same table name structure. Tables in different schemas are not isolated from each other; for example, they can be joined as readily as tables that belong to the same schema.
