there's knowing sql and then knowing sql. it's one thing to be able to do simple crud operations with sql, but stuff like the following is much rarer and more valuable: - complex aggregations (aggregate() filter (where ...)) - window functions (row_number() over (partition by ...)) - ctes (with some_query as (...) select from some_query ...) - using exists properly in where clauses for performance - knowing which types of where clauses are detrimental to performance - knowing the details of how various combinations of joins will improve or harm performance - knowing how to properly design schemas - knowing how to correctly index tables for the queries used