Core querying
Be comfortable with SELECT, filtering, CASE expressions, aggregation, joins, NULL handling, date functions, and common table expressions. Accuracy matters more than cleverness.
Window functions
Practice ROW_NUMBER, RANK, LAG, LEAD, and rolling aggregates. These patterns are common in retention, funnel, cohort, and period-over-period analysis.
Data quality and debugging
Check row counts, uniqueness, missing values, join cardinality, and unexpected duplication. Explain how you validated a result before sharing it.
Performance awareness
Learn to read a query plan at a basic level, filter early when appropriate, avoid unnecessary columns, and understand why indexes and table grain matter.