How Can I Efficiently Join Tables Across Multiple SQLite Databases?

Joining Tables from Multiple SQLite Databases

Joining tables from different databases can be a complex task, but SQLite offers a convenient solution with the ATTACH keyword. By attaching additional database files to the current connection, you can seamlessly integrate data and perform cross-database queries.

Attaching Multiple Databases

To attach a database file, use the following syntax:

ATTACH 'database1.db' as db1;
ATTACH 'database2.db' as db2;

This will attach two databases named 'database1.db' and 'database2.db' to the current connection, using the aliases 'db1' and 'db2' respectively.

Listing Attached Databases

To view all connected databases, use the '.databases' keyword:

.databases

Performing Cross-Database Queries

After attaching the databases, you can perform cross-database queries:

SELECT
  *
FROM
  db1.SomeTable a
INNER JOIN
  db2.SomeTable b ON b.SomeColumn = a.SomeColumn;

This query will join tables 'SomeTable' from both databases 'db1' and 'db2' based on the 'SomeColumn' column.

Considerations

• The 'main' and 'temp' database names are reserved for the primary database and temporary data, so they should not be used for attachments.
• The maximum number of databases that can be attached is determined by the SQLITE_MAX_ATTACHED compile-time setting, which defaults to 10.
• Using attached databases can affect performance, so consider caching or pre-computing joins for optimal efficiency.

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