How to Recover Data From a Corrupted SQL Database?

Have you lost your data due to a corrupted backend? Don’t worry, it is not completely lost; it can be recovered by following certain methods of data recovery from the SQL database. The recovery of data from a corrupted SQL database involves assessing the damage level, starting with restoring from the latest clean backup. If the backups are unavailable, run the command DBCC CHECKDB with repair options. For another method of recovery, you can use third-party SQL recovery software to fix damaged files, tables, views and stored procedures. Let’s discuss in detail the major methods to recover data from a corrupted SQL database in this blog. 

Here is the comprehensive guide to recovering data from a corrupted SQL database.

• Initial assessment and preparation

Before attempting any repair, take the following steps to prevent more data loss.

1. Immediately stop the SQL server service, as this prevents further writing to the corrupted files.
2. Create a backup copy of the corrupted primary data file and transaction log file immediately. 
3. Examine the SQL server error logs and Windows event logs to find the exact cause and region of the corruption.

• Restore from backup

This is one of the most preferred methods. If you have a valid, uncorrupted backup, this is the safest method. 

1. Open SQL Server Management Studio
2. Right-click the corrupted database, select tasks, and then restore the database. 
3. Select the last known good backup. 

• Using DBCC CHECKDB when no backup is available   

If you do not have backups, you can use the DBCC CHECKDB command to detect and fix errors. The step of command you can run for this is as follows:

1. Set the database to EMERGENCY mode to make it read-only.
2. Set single-user mode, as it ensures exclusive access
3. Run a repair with data loss to fix corruption. 
4. Revert to multi-user mode.

• Rebuilding the transaction log

If the database is in RECOVERY PENDING state due to a corrupt transaction log, you can rebuild it. 

The steps to follow include:

1. As already mentioned above, set the database to emergency and single-user mode. 
2. Then rebuild the log. If this fails, you need to use DBCC CHECKDB with Repair to allow data loss.    

• Third-party SQL recovery tools

When DBCC CHECKDB fails or causes too much data loss, using third-party specialised software is the best option. For this scan, the corrupted files, preview database objects before recovery, recover data from corrupted LDF files and at last recover all the deleted records.

• Post-recovery steps

After the database is recovered:

1. Run DBCC CHECKDB again without repair options to ensure the database is not consistent, as it is important to verify the data integrity.
2. Immediately perform a full backup of the repaired database.
3. Most of the corruption is caused by the I/O subsystem failures, so properly performing a hardware diagnosis is necessary to prevent the issue. 

Conclusion

Recovering a corrupted SQL database is an essential task that makes it necessary to follow a structured approach, which prioritises data integrity over speed. The most reliable method is recovering from a recent backup; however, when backups are missing or corrupted, DBCC CHECKDB offers a powerful manual repair mechanism that may cause data loss. The commands like repair_allow_data_loss and repair_rebuild are the essential tools in a database administrator’s collection. They are helpful in repairing structural corruption, but they should be used with caution. When these built- in methods are ineffective, turning to professional third-party SQL recovery software becomes the most reliable alternative to save tables, views and stored procedures. At iTechnology Australia, we help our clients to effectively implement a tested backup strategy comprising all the backups, which is the most effective way to protect against data loss, as advised by our expert Mr. Sella Silva.    

  FAQs

• Can I repair the database without losing any data?

Yes, if you have a healthy backup or if the corruption is minor, it can be fixed with the command repair_rebuild without losing any data.

• What causes SQL database corruption?

Most of the database corruption is caused by the I/O subsystem issues, such as failing hardware, faulty disk drivers or power outages during write operations.