What is "query containment"? Any one knows what is "query containment" and where do we use it please?
As far as I can search on Google, I only found one here. 

Although, I've got some idea on the subject but I'm finding it difficult to understand the use case of it.

Many thanks,
The linked paper has a definition:


  In the relational model, a query Q1 is said to be contained in the query Q2 if Q1 produces a subset of the answers of Q2, for any given database.