Confirm the fault type; 2. Restore with backup; 3. Use DBCC CHECKDB to repair corruption; 4. Set up monitoring and backup mechanisms. When encountering a SQL Server database failure, first determine whether it is a system crash, database corruption, artificial deletion or hardware problem, and check the error log and event viewer to narrow the scope of troubleshooting; if there are complete, differential and transaction log backups, you can restore and use the WITH REPLACE parameters in turn; if the database is suspected to be damaged, run DBCC CHECKDB to check and rebuild the index according to the results or repair it carefully; in order to prevent problems before they happen, automatic backup, integrity check, disk monitoring and alarm mechanisms should be set up to ensure that data can be quickly restored at critical moments.

If there is a problem with the database, recovery is the key. Once a SQL Server database fails, the most important thing is to quickly determine the type and take correct measures to minimize data loss and downtime. Don't rush to do it, first figure out what you are facing.

1. Confirm the fault type: is it crash, damaged or deleted by mistake?

The first step in SQL Server failure recovery is to identify the source of the problem. Different types of problems require different coping strategies:

• System crash or service failure to start : it may be a configuration error, insufficient resources (such as insufficient memory), or a corruption of the log file.
• Database corruption : Usually manifested as page verification failure, index corruption, etc., which can be found through DBCC CHECKDB inspection.
• Human error operations : such as accidentally deleting tables or updating data, it depends on whether there is a backup or transaction log that can be rolled back.
• Hardware failure or disk problems : This type of problem may affect the entire instance or even multiple databases, making recovery difficult.

When encountering problems, first look at the SQL Server error log (Error Log) and Windows Event Viewer, which can help you narrow the scope of troubleshooting.

2. Using backup is the most direct way to restore

If you have regular backups of complete differential transaction logs, recovery is much easier. The recovery process is roughly as follows:

• Find the most recent full backup;
• If anything, apply the most recent differential backup;
• Finally, restore to the last available point in time through transaction log backup.

Note: During the recovery process, if the target database is in use, remember to add the WITH REPLACE parameter, otherwise an error may be reported because of the existence of the database with the same name.

For example: Suppose you made a full backup at 8 a.m., and there was a transaction log backup every hour, and a serious error occurred at 9:45. You can start the restore from the full backup at 8 o'clock, and then apply the log backups at 9:00, 9:15, 9:30, and 9:45 in turn until the data state is approached before the failure.

3. In the face of database corruption, try DBCC CHECKDB and repair options

When suspected that the database is corrupt, run DBCC CHECKDB('YourDBName') WITH NO_INFOMSGS, ALL_ERRORMSGS to check the problem immediately.

According to the output results, common processing methods include:

• If it is just a non-clustered index corrupt, you can directly rebuild the index;
• If the system table is damaged, it will be more troublesome. You may need to enter single-user mode and try to repair it;
• Using REPAIR_ALLOW_DATA_LOSS is the last choice, but it may lead to data loss and must be operated with caution;
• Before repairing, be sure to back up the current database status, even if it is "broken".

Sometimes, even if DBCC reports an error, the database can continue to run, but don't take it lightly and arrange a repair as soon as possible.

4. Prevent problems before they happen: Set up monitoring and automatic backup mechanisms

Many failures can actually be prevented in advance. for example:

• Set up SQL Agent Job to perform integrity checks regularly;
• Automatic backup using maintenance plan or Ola Hallengren script;
• Monitor the growth of disk space and log files;
• Turn on the database email notification function and remind you as soon as possible if you have any questions.

In addition, some common faults can be simulated in the test environment for drilling, so that if there is any problem, you won’t be in a hurry.

Basically that's it. SQL Server recovery is not too difficult, but there are many details and the steps are critical. You must think clearly about the consequences at each step. Only by doing backup and monitoring at any time can you have a clear mind at critical moments.

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