Data SQL Skill

Version: 1.0 Stack: SQL (PostgreSQL, Redshift, Athena, Spark SQL)

SQL is the only language where a working query can be 1,000x slower than an equivalent working query. A missing index turns a 50ms lookup into a 50-second full table scan. SELECT * breaks downstream code when someone adds a column. Correlated subqueries silently execute N+1 patterns that disappear in development and cripple production. The query planner doesn't warn you — it just does what you asked.

SQL written with index awareness, explicit columns, and CTEs is both faster to execute and easier to debug. Clarity and performance reinforce each other.

Scope and Boundaries