What is restoration in SQL Server?

Restoration in SQL Server refers to the process of restoring or recovering lost or corrupted data from a backup. SQL Server provides several methods for restoring data such as full database restoration, differential database restoration, transaction log restoration, file and file group restoration, and page restoration. The restore process in SQL Server retrieves backups made earlier and applies them to recreate the state of data at the time the backup completed.

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Why is restoration needed in SQL Server?

Restoration is needed in SQL Server when data gets corrupted, deleted, or otherwise lost. Some common scenarios where restoration is required are:

Hardware failure – If a disk drive fails causing data files to become inaccessible or corrupted.

Software corruption – Bugs, errors or failures in SQL Server can lead to corruption of database files.
Human errors – Administrators dropping databases, tables or rows by mistake.
Malicious attacks – Hackers gaining access and manipulating or deleting data.

Application errors – Bugs in an application corrupting data.
Disasters – Data center outages, fires or natural disasters destroying servers and data.

In all such cases, restoration provides a way to recover data from an earlier point in time when it was backed up and was still accessible.

What are the prerequisites for restoration in SQL Server?

The key requirements for being able to restore data in SQL Server are:

Backups – Database backups made earlier must be available to restore from. The more recent the backup, the more data can be recovered.
Backup media – The backup files must be available on disk, tape or cloud storage depending on where backups were written.

Master database – The master database must be operational since it stores metadata about backups.
User access – The user performing the restore must have sufficient permissions to carry out the restore operation.

Additionally, the SQL Server services must be up and running for restoration to be feasible. The databases being restored can be offline.

What are the different types of restore operations?

SQL Server supports various types of restore operations depending on the situation and how much data needs to be recovered. The main restore types are:

Full database restore

Restores the entire database from backup files. All data files, log files and other objects in the database are recovered to the point in time of the backup.

Differential database restore

Restores the database from the most recent full backup and then the latest differential backup. Differential backups contain only data changed since the last full backup.

Transaction log restore

Recovers data to a specific point in time by applying transaction log backups after a full or differential restore. Log backups contain details of all modifications.

File and file group restore

Restores only a specific file or file group within the database that needs to be recovered.

Page restore

Restores individual pages that have become corrupted rather than the entire database. Available only in Enterprise Edition.

The type of restore operation to use depends on factors such as how much data needs to be recovered, how recent it needs to be and how much time the restore can take. A larger restore scope usually means more time for the operation.

What are the general steps to restore a SQL Server database?

These are the usual high-level steps to carry out a database restore in SQL Server:

Identify the database backup files required for the restore operation.

If needed, move the backup files to the SQL Server instance location.
Verify that the backup files are valid and can be restored.
Write and test the restore scripts.

Disconnect users and applications from the database being restored.
Restore the database using the required restore type.
Recover the database to the desired point in time.

Run DBCC CHECKDB to validate database consistency.
If satisfied, bring the database online for usage.
Reconnect users and applications to the restored database.

The exact steps can vary based on the specific environment, recovery requirements and backup media.

How does full database restoration work?

Full database restore recovers the entire database to the point in time when the backup completed. Here are key details about full database restore in SQL Server:

A full database backup contains all data files, log files and database objects at backup time.

The RESTORE DATABASE command is used specifying the database name and backup location.
All existing database files are overwritten with the full backup contents.
The database is recovered to the point when the backup completed.

Additional transaction log backups can be applied for point-in-time recovery.
Takes more time to complete than differential or log restores.

Full restore is used when an entire database needs to be recovered or recovered to an earlier point in time.

Full Database Restore Example

This T-SQL statement restores the MyDB database from a full backup stored at disk location ‘C:\backups\MyDB.bak’:

RESTORE DATABASE MyDB 
FROM DISK = 'C:\backups\MyDB.bak'
WITH RECOVERY

How does differential database restoration work?

Differential database restores use a combination of the last full backup and the latest differential backup. Here are key points about differential restores:

A differential backup contains data changed since the last full backup.

Needs the full backup and latest differential backup available.
RECOVERY option replays log backups taken after the differential backup.
Much faster than full restore when frequent full backups are not taken.

Multiple differential backups cannot be applied together.

Differential database restore is useful when frequent full backups are not feasible such as with large or highly active databases.

Differential Database Restore Example

This T-SQL script restores MyDB first using its latest full backup and then the most recent differential backup:

RESTORE DATABASE MyDB
FROM DISK = 'C:\backups\MyDB_full.bak'

RESTORE DATABASE MyDB
FROM DISK = 'C:\backups\MyDB_diff.bak'
WITH RECOVERY

How does transaction log restoration work?

Transaction log restore applies transaction log backups to recover a database to a specific point in time. Key characteristics include:

Transaction logs record all data modifications in the database.
Taken frequently to minimize data loss.

Log restores “roll forward” transactions to a specific time.
Allows restore to precise seconds and milliseconds.
Requires log chain with no missing backups.

Transaction log restore is vital for recovering data to just before a data loss or corruption event when point-in-time recovery is needed.

Transaction Log Restore Example

This T-SQL script restores MyDB database and transaction logs to 8:35 AM on 5/10/2022:

RESTORE DATABASE MyDB  
FROM DISK = 'C:\backups\MyDB.bak'

RESTORE LOG MyDB
FROM DISK = 'C:\backups\Log_1.bak'

RESTORE LOG MyDB  
FROM DISK = 'C:\backups\Log_2.bak'
WITH STOPAT = 'May 10 2022 8:35 AM'

How does file and file group restoration work?

File and file group restore targets only a specific file or set of files to be recovered instead of the full database. Key points:

SQL Server databases have individual data and log files.
Databases can have file groups containing multiple files.
Only needed files or file groups are restored.

Much faster recovery of subsets of data.
Read-only file groups can stay online during restore.

File and file group restore is helpful when only a portion of a large database needs to be recovered quickly.

Filegroup Restore Example

This example restores only the PERSONNEL filegroup of the MyDB database:

RESTORE DATABASE MyDB
FILEGROUP = 'PERSONNEL'
FROM DISK = 'C:\backups\MyDB_Personnel.bak'
WITH RECOVERY

How does page level restoration work?

Page restore targets recovery of only damaged or missing pages rather than entire data files. Key characteristics:

Available in SQL Server Enterprise Edition only.

Individually corrupted pages can be precisely restored.
Pages must be identified using DBCC CHECKDB.
Much smaller restore scope than files or file groups.

No downtime needed if page can be repaired online.

Page level restore provides granular recovery when corruption is limited to specific pages only.

Page Restore Example

This T-SQL statement restores page 15 of the Users table in MyDB database:

ALTER DATABASE MyDB  
SET SINGLE_USER

RESTORE DATABASE MyDB
PAGE = '15:122013' 
FROM MyDB_Backup

ALTER DATABASE MyDB
SET MULTI_USER

What are some common best practices for restoration?

Some best practices to follow when planning and performing restore operations in SQL Server:

Take frequent full and transaction log backups of databases and test restores.
Ensure backup files are stored securely on separate devices.

Document all recovery configurations and policies.
Assign restore roles and grant appropriate permissions.
Monitor disk space for backup storage locations.

Keep older backups for extended recovery windows.
Validate all backups using RESTORE VERIFYONLY.
Keep backup metadata in the msdb database.

Automate backup and restore processes when possible.

Careful planning, preparation and testing of database restores enables reliable recoverability when disasters occur.

What are some common challenges with SQL Server restoration?

Some typical difficulties faced with restore operations in SQL Server environments:

Unavailable or corrupt backups – Unable to restore without backups.
Incomplete backup chain – Missing transaction log backups break restore sequences.
No validated test of restoration – Restore issues are found only when disasters strike.

Unsupported backup types – Backup types not compatible with current SQL Server version.
Inadequate permissions – Users lack sufficient rights to perform the restore.
Too many manual steps – Heavily manual processes leading to errors.

Backup storage bottlenecks – Slow backup devices and media affecting restore speeds.
No documented procedures – Lack of documentation on restore process and settings.
Unrealistic recovery time expectations – Large databases take time to restore.

Careful planning and testing helps anticipate and address potential restoration issues before they result in extended downtimes.

What are the key differences between full, differential and transaction log backups?

Backup Type	Full Backup	Differential Backup	Transaction Log Backup
Contents	Entire database	Data changed since last full backup	Transaction log records since last log backup
Frequency	Daily or weekly	Daily	Every few minutes
Recovery Needs	Full or differential backups	Full + latest differential backup	Full or differential + subsequent log backups
Speed	Slowest	Faster than full	Very fast
Data Loss	All changes since last full backup	Changes since last differential backup	Minimal data loss

Full backups capture the entire database state while differentials and logs capture subsequent changes for point-in-time recovery.

Conclusion

Restoration is a critical process in SQL Server environments to recover from data loss due to corruption, human errors or disasters. SQL Server provides robust restore capabilities including full, differential, log, file group and page level recovery. Careful planning and testing of backup and recovery configurations is essential to handle realistic restore scenarios. Following SQL Server best practices for backup processes, testing restores and monitoring the backup environment helps minimize both the frequency and impact of restore situations.