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In today's data-driven world, information is often distributed across multiple databases. This increases the complexity of querying data, as you may need to retrieve information from more than one database at a time. In SQL, this is accomplished via cross-database joins. However, with this increased complexity comes an elevated risk in security. In this article, we will explore the cross-database joins and security considerations in SQL Server 2012, SQL Server 2014, MySQL 5.6, DB2 10.5, and Azure SQL.

In SQL Server 2012 and 2014, you can perform cross-database joins by using fully qualified names. However, this requires the user to have appropriate permissions on both databases. This can be a security concern because it potentially opens up more data to the user than they should have access to. Here's an example of a cross-database join:

SELECT * FROM Database1.dbo.Table1
JOIN Database2.dbo.Table2
ON Database1.dbo.Table1.ID = Database2.dbo.Table2.ID

MySQL 5.6 also supports cross-database joins, but the user must have SELECT privilege on both databases. Although this can potentially give users access to more data than they should have, MySQL has an advantage in security because it supports row-based access control. This allows the database administrator to limit access to certain rows of a table, which can reduce the risk of unauthorized data exposure.

SELECT * FROM database1.table1
JOIN database2.table2
ON database1.table1.id = database2.table2.id;

DB2 10.5 also supports cross-database joins with a feature known as "Federation". This allows users to access data from multiple databases as if it were from one database. However, this can be a security concern as it potentially exposes more data to the user. As a security measure, DB2 10.5 has role-based access control, which allows the database administrator to create roles with specific privileges and assign them to users.

Azure SQL, Microsoft’s cloud-based relational database system, allows cross-database queries with the use of elastic database query. However, permissions need to be carefully managed to ensure data security. Azure SQL provides features like firewall rules, authentication and authorization mechanisms, and threat detection alerts to ensure that your data remains secure.

In conclusion, while cross-database joins can increase the efficiency of data retrieval, they can also present security challenges. By understanding these challenges and how to manage them, you can ensure that your data remains secure while still benefiting from the power of cross-database joins. Always remember that the most secure system is the one that grants the least privileges necessary for a task, so ensure you review your users' permissions regularly.