Solving the H2 Database Issue: How to Access Multiple Databases in One Query

Are you struggling to query data from multiple databases in H2? Learn how to work around H2's limitations when testing against multiple MSSQL databases within a single query.
---

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Select in H2 but with database prefix

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Troubleshooting H2 Database Queries for Multiple MSSQL Databases

When working with databases in a professional setting, it's common to run into obstacles that hinder progress. Recently, I faced a challenge at work that required querying data from multiple MSSQL databases hosted on the same machine. This query functioned perfectly in production. However, when I transitioned to testing with a mock H2 database, I encountered a roadblock. The error message "Schema productDb not found" threw a wrench in my end-to-end testing process, indicating that H2 did not support my approach to database schema referencing. The crux of the problem: how can I efficiently query across multiple databases in H2 without resorting to multiple calls and merging the results later?

Understanding the H2 Database Environment

H2 is a lightweight, open-source relational database management system written in Java. It’s often preferred for testing due to its simplicity and the ability to run in-memory without the complexity of more substantial systems.

However, H2 has specific limitations, especially when it comes to referencing multiple databases. To address my specific issue, I need to explore a couple of viable solutions to help you bypass the constraints of H2.

Suggested Solutions

Let’s discuss the approaches you can take to navigate querying multiple databases through H2 successfully.

1. Utilize a Single H2 Database

Steps:

Create all tables in one H2 database.

Modify your JDBC connection URL by appending ;IGNORE_CATALOGS=TRUE.

This adjustment allows H2 to treat all tables as being within the same catalog, thereby eliminating the schema errors that occur when attempting to access distinct databases.

2. Create Linked Tables

Alternatively, H2 supports the use of linked tables. You can maintain two separate databases and create links for each table using the CREATE LINKED TABLE command.

Keep in Mind:

While this method is viable, linked tables may exhibit slower performance, which could affect your tests' speed.

3. Adjust Database Schemas

If you find yourself with tables that share the same names across the databases, distinguishing between them in H2 can be problematic. To resolve this, you could use different schema names within the single H2 database.

Example:

For instance, you can structure your tables as follows:

This differentiation ensures that H2 can correctly reference each table without confusion.

4. Consistency in Testing Environments

Regardless of the approach you choose, it’s vital to maintain consistency in your testing environment. Ideally, using the same Database Management System (DBMS) across production and testing phases helps reduce potential incompatibilities.

Key Takeaway:

Utilizing a consistent environment minimizes surprises during deployment as code that functions correctly in testing should ideally behave in the same manner in production.

Conclusion

Navigating database queries in H2 when dealing with multiple sources can be challenging but is manageable with the right strategies. Whether you decide to combine everything into a single database, utilize linked tables, or adjust your schemas, being aware of these options empowers you to structure your testing environment effectively and efficiently.

In summary, while H2 might not natively support complex database references as MSSQL does, with a little ingenuity and adjustment, you can create a functional setup for your testing needs.