In addition to the user-defined table, we have the option to work with various unique sorts of tables in addition to the fundamental user-defined table. System Tables are a unique class of tables that the SQL Server Engine uses to hold data that can be accessed through system views regarding the configurations and objects of SQL Server instances.
The underlying tables that hold the metadata for a certain database are called system tables. Because it has some extra tables that are not present in any of the other databases, the master database is unique in this regard. These tables have server-wide persisted metadata in them.
The system catalog views allow you to browse the data in the system tables. System tables can’t be manually updated. Users shouldn’t make direct changes to the system tables. For instance, avoid using user-defined triggers, DELETE, UPDATE, or INSERT statements to change system tables.
Microsoft advises against making direct queries to the system tables. The architecture of SQL Server’s foundation determines the structure of the system catalog tables, which varies from version to version. If a program directly queries system tables that change or are absent in the new version of SQL Server, it may be necessary to at least partially rewrite the program when migrating to the new version.”
Microsoft keeps these components compatible from one release to the next. The core architecture of SQL Server determines how the system tables are formatted, which might vary from version to release. As a result, before using a subsequent version of SQL Server, programs that directly access the undocumented columns of system tables may need to be modified.
The tables that SQL Server keeps track of include details about all the objects, data types, constraints, configuration settings, and resources that are at its disposal. System tables are another name for this collection of tables. A subset of tables exists in every database (including the Master database) and provides information about the objects and the other subset of tables resides solely in the Master.
SQL Server’s system tables hold the crucial metadata or information about your data. Table names, column names, and data type details are included in this data so that SQL Server can execute queries and produce result sets in an appropriate manner. System tables contain data about legitimate users and their permissions, ensuring the security of data, as well as data on your SQL Server configuration, allowing you to forecast and manage system behavior. You can develop better solutions by comprehending the data in these tables and by learning why SQL Server performs the way it does. Most often, a system table can be identified by its name. The three letters sys are the beginning of every system table, although not every word that begins with sys is a system table. System tables can also be recognized by their object IDs, which are all fewer than 100.
Different types of system tables in SQL:
- sys.objects – exists in the master database only, and displays each object, type, and created date.
- sys.indexes – exists in the master database only, displaying each index and type.
- sys.sysschobjs – exists in every database, each row displays an object in the database.
- sys.sysbinobjs – exists in every database, and displays a row for each Service Broker entity in the database.
- sys.sysnsobjs – exists in every database, and displays a row for each namespace-scoped entity. This table is used for storing XML collection entities.
- sys.sysdbreg – exists in the master database only, displaying a row for each registered database.
- sys.sysxsrvs – exists in the master database only, displays a row for each local, linked, or remote server.
- sys.sysrmtlgns – exists in the master database only, displays a row for each remote login mapping, used to map logins that claim to be incoming from a corresponding server to an actual local login.
- sys.syslnklgns – exists in the master database only, and displays a row for each linked login mapping. Remote procedure calls and distributed queries that travel from a local server to a corresponding linked server use linked login mappings.
- sys.sysxlgns – exists in the master database only, and displays a row for each server principal.
- sys.sysdbfiles – exists in every database. If the column dbid is 0, the row presents a file that belongs to the database.
- sys.sysusermsg – exists in the master database only, each row displays a user-defined error message.
- sys.sysobjkeycrypts – exists in every database, and displays a row for each symmetric key, encryption, or cryptographic property associated with an object.
- sys.syslogshippers – exists in the master database only, and displays a row for each database mirroring witness.
- sys.sysendpts – exists in the master database only, and displays a row for each endpoint created in the server.
- sys.sysobjvalues – exists in every database, and displays a row for each general value property of an entity.