SQL

SQL, the essential language whenever you need to query data. We cover queries and their building blocks — filters, joins, aggregations, subqueries, window functions — then everything that separates a query that works from a query that holds in production: indexes, execution plans, volume. Articles offer reproducible tutorials on PostgreSQL and other relational engines, plus the interview questions that come up most often, with the expected reasoning rather than only the answer. Strong SQL mastery remains one of the highest-ROI skills for a data profile.