Hi @Chirag Sachdeva ,
REPAIR_FAST
Maintains syntax for backward compatibility only. No repair actions are performed.
REPAIR_REBUILD
Performs repairs that have no possibility of data loss. This can include quick repairs, such as repairing missing rows in nonclustered indexes, and more time-consuming repairs, such as rebuilding an index.
This argument does not repair errors involving FILESTREAM data.
Backup is very important, restore the backups is the best choice for corrupt databases. You can using the REPAIR_ALLOW_DATA_LOSS option as a last resort when you cannot restore a database from the backup. If you do not have a backup and cannot risk losing data, you can use a third part SQL repair software to repair the database without any loss in database integrity.
For detail information about DBCC CHECKDB, MS document DBCC CHECKDB (Transact-SQL) is the best choice.
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