In the SQL environment Catalog and Schema are all abstractions, mainly used to resolve naming conflicts
A database system comprising a plurality Catalog, Catalog each comprising a plurality of Schema, Schema each comprising a plurality of database objects (tables, views, field, etc.)
The fully qualified name of the database table objects may be expressed as: Cataog name . . The Schema Name Table
SQL standard does not require the fully qualified name of each database object is unique, just like the domain name, if you like, each IP address can have multiple domain names
From an implementation perspective, a variety of databases and systems Catalog Schema support and implementations vary widely, relatively simple and commonly used way is implemented as a Catalog name database name, user name as Schema names, specifically referring the table below:
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