What is DBCC?

DBCC (Database Consistency Checker Commands) is used to check logical and physical consistency of database structure.DBCC statements can fix and detect problems.They are grouped in to four categories
Maintenance commands like DBCC DBREINDEX , DBCC DBREPAR etc , they are mainly used for maintenance tasks in SQL SERVER.
Miscellaneous commands like DBCC ROWLOCK , DBCC TRACEO etc , they are mainly used for enabling row-level locking or removing DLL from memory.
Status Commands like DBCC OPENTRAN , DBCC SHOWCONTIG etc , they are mainly used for checking status of the database.
Validation Commands like DBCC CHECKALLOC,DBCC CHECKCATALOG etc , they perform validation operations on database.
Note :- Check MSDN for list of all DBCC commands, it is very much possible specially during DBA interviews they can ask in depth individual commands.
Below is a sample screen in which DBCC SHOWCONTIG command is run. DBCC SHOWCONTIG is used to display fragmentation information for the data and indexes of the specified table.In the sample screen gCustomerh table is checked for fragmentation information. If you look at the figure “Scan density” is 100 if everything isucontigious in this image it is 95.36% which is decent percentage. So such type of useful information can be collected by DBCC command and database performance and maintenance can be improved.