I learned data modeling formally (Entity-Relationship Diagrams, cardinality, indices, referential integrity, normalization) and have been using these golden principles throughout my career. It's what enables an app to say, for example, "show me all the products across all regions that were purchased by consumers making more than X amount a year, who are also subscribers to Y service", or "show me all courses taught by Instructor A and which are attended by Student B".

One common problem I've seen throughout my career, in older apps where good data-modeling principles were not known or applied, is the problem of "slow queries". One effective solution is to add table indices, and doing this well is both an art and science. Best to build new tables with these indices in place for fast lookups and to avoid this creeping problem.

Other good principles include designing database tables correctly, to reflect object properties and relationships accurately. Remember, we are representing real-world objects in the virtual world. Too often, many developers treat database tables as if they were spreadsheets!