SQL

Write complex SQL joins (inner, outer, cross, self-joins)

Use window functions (ROW_NUMBER, RANK, LAG, LEAD)

Write CTEs and recursive queries for hierarchical data

Optimize SQL queries with EXPLAIN, indexes, and query rewriting

Create pivot tables and dynamic crosstab queries

Explain a complex SQL query in plain English, line by line

Identify and fix SQL injection vulnerabilities in code

Write reversible database migrations with up/down scripts