Can we use join for two different database tables?

sql

cross-database-joins

sql-server

database-design

byAnton Shumikhin·Dec 15, 2024

Yes, you can! In SQL, you can perform JOIN operations across different databases housed on the same server. This involves using fully qualified names (database.schema.table) in your queries. Here's a short and crisp demonstration:

// Eddie Apropos: "Hey SQL, do you believe in life after joins?"
// SQL: "Yes, but only for SELECT clause and friends."

SELECT 
  db1.Table1.column1, 
  db2.Table2.column2 
FROM 
  db1.Schema.Table1 
JOIN 
  db2.Schema.Table2 
ON 
  db1.Table1.id = db2.Table2.foreign_id;

Here, db1 and db2 represent different databases; Schema usually is dbo; Table1 and Table2 are respective tables joined using common identifiers: id from Table1 and foreign_id from Table2.

Checklist for successful Cross-Database Joins

Performing cross-database joins require few considerations. Make sure:

Permissions to both databases are granted for your user account.
You consider setting synonyms for often joined tables. It simplifies your queries.
Performance aspects are checked. Cross-database joins might differ in efficiency compared to same database joins.

Embracing Synonyms: Simplifying Cross-Database Joins

Synonyms, a feature in SQL Server, can greatly simplify the use of cross-database joins. They enable you to use aliases for databases, tables or even columns, thus avoiding repeatedly qualifying names in every query.

// Codemaster: "What's in a name? That which we call a table, by any other name would smell as sweet."

CREATE SYNONYM dbo.Table1 FOR db1.Schema.Table1;
CREATE SYNONYM dbo.Table2 FOR db2.Schema.Table2;

// SQL server: "You've got a friend in me...alias me anytime."

SELECT 
  Table1.column1, 
  Table2.column2 
FROM 
  dbo.Table1 
JOIN 
  dbo.Table2 
ON 
  Table1.id = Table2.foreign_id;

Leveraging synonyms promotes cleaner syntax and easy maintenance of database queries.

Goof-ups to avoid with Cross-Database Joins

Compatibility: Different DBMS might have different query syntax.
Data Consistency: Upholding integrity can turn Herculean with multiple databases cross-joins.
Backup and Recovery: Watch out for cross-database dependencies that can complicate your backup and recovery processes.

Harnessing Linked Servers for complex architectures

In cases where tables are located not only on different databases, but also on different SQL Server instances, you can consider using linked servers. With linked servers, a server can access another server's data enabling cross-instance joins.

// Joe Coder: "I'm the link to your server, you never knew you missed."

SELECT 
  localDb.dbo.Table1.column1, 
  remoteDb.dbo.Table2.column2 
FROM 
  localDb.dbo.Table1 
JOIN 
  [LinkedServerName].remoteDb.dbo.Table2 
ON 
  localDb.dbo.Table1.id = remoteDb.dbo.Table2.foreign_id;

This presents massive possibilities for data integration, albeit with a careful watch on security and performance.

Common Pitfalls you can dodge

Transactions: Keep an eye out for transactions spanning cross-database joins. They could lead to complex rollbacks upon failure.
Data Distribution: If databases are geographically distributed, network latency could hamper query performance.
Collation variance: Databases with different collations can pose glitches in joins due to collation conflicts. Needs explicit handling!

Workarounds and Best Practices for sophisticated scenarios

Federated Databases: PostgreSQL's foreign data wrappers offer a fine alternative for cross-database queries.
Database Links: Oracle enables setting up database links, facilitating cross-database operations.
Sharding Patterns: Employing sharding strategies tend to split databases. This requires careful design to support joins.