When it comes to working with databases, there are various concepts and terms that can sometimes be confusing, especially if you are transitioning from a different database management system. One such topic that often raises questions is the difference between instances and databases in SQL Server.

During a recent conversation with a DBA who had extensive experience with Oracle and was in the process of transitioning to SQL Server, we discussed the fundamental differences between instances in Oracle and SQL Server, and when to use each. This conversation sparked my interest, and I wanted to share the insights with you.

In Oracle, the relationship between instances and databases is straightforward – one instance equals one database, which can contain multiple schemas. On the other hand, in SQL Server, one instance can have several databases, each of which can also have multiple schemas. Additionally, SQL Server has built-in databases such as master, model, msdb, and tempdb, which store essential schema information.

When migrating from Oracle to SQL Server, it is crucial to consider how you want to handle the schemas. You can either map Oracle schemas to SQL Server schemas or move them to separate SQL Server databases. There is no general rule of thumb for this decision, but here are a few factors to consider:

• If the schemas do not interact much, it may be best to use separate databases in SQL Server.
• If the schemas interact frequently, it might be more efficient to migrate them to different schemas within a single SQL Server database.

By using the above decision tree, you can simplify the process of determining the best approach for your specific scenario.

Lastly, it is important to note that in SQL Server, it is advisable to avoid using the default/dbo schema. Instead, it is better to create new schemas that are relevant to your job or project. This allows for better organization and management of database objects.

If you want to dive deeper into the importance of database schemas in SQL Server, I have written an article on the topic that you may find helpful. You can find it here.

Understanding the differences between instances and databases in SQL Server is essential for anyone working with this database management system. By grasping these concepts, you can make informed decisions when it comes to designing and managing your databases.