The systems that support it are described in the previous table.Back to Techniques The most straightforward approach is to manually combine the data from the sources.This article covers the various options for querying and reporting data from multiple sources and different techniques and software that is available. The first task is to determine what "multiple database" means in your particular situation as that determines the options available.There are three key scenarios: This query is identical to a query where all tables are in a single database except for providing the database name in the table alias (in bold).It is very common that the data needed for a report, business decision, or application is spread across several data sources which may be relational databases, spreadsheets, documents, No SQL sources, or "Big Data" sources.

Back to Scenarios The second scenario is when you have multiple databases that reside on different servers but the database software is the same on all servers.

Back to Techniques A common solution to handle multiple data sources is to produce a single data warehouse that contains data from multiple sources.

This data has been extracted from its original source, translated, summarized, and aggregated into a form suitable for efficient querying and reporting, and loaded into another database.

In some situations, rather than creating separate databases you can create one database with multiple schemas that has the same effect.

In summary, if all your databases are on one server instance, then multiple database querying is as easy as prefixing the table name with the database or schema name depending on your database software.