Query Languages for Data

When the relational model was introduced, it changed how we query data from imperative to declarative.

Declarative vs. Imperative

• Imperative: You tell the computer how to perform a task (step-by-step logic, loops).
• Declarative: You specify the pattern of the data you want, but not how to achieve it.

The CSS Analogy

Declarative languages are not limited to databases. Consider a web browser:

• Declarative (CSS): li.selected > p { background: blue; }. You describe what should be blue. The browser handles the implementation and updates.
• Imperative (JavaScript): You would have to loop through all li elements, check for the class, find the child p, and set the color. If the class changes later, you must manually update it again.

MapReduce Querying

MapReduce is a mix: the logic is expressed with snippets of code (map and reduce), but the framework handles the execution across machines. It is lower-level than SQL but more flexible for certain tasks.