Early applications usually only connected to one database, and calculations were also performed in the database, with little or no problem with mixed calculations from multiple data sources. The data sources of modern applications have become very rich, and the same application may also access multiple data sources, such as various SQL and NoSQL databases, text/XLS, WebService/Restful, Kafka, Hadoop…. Mixed computing on multiple data sources is an unavoidable problem that needs to be addressed.
Implementing direct hard coding in applications is very cumbersome, and commonly used application development languages such as Java are not good at doing such things. Compared to SQL, their simplicity is far inferior.