SQL
The 50-year-old skill every data job still screens for.
SQL in depth: querying and filtering, joins, aggregation, subqueries and CTEs, window functions, schema design, indexes and data modification - with practice resources on every topic.
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- 1
Query Foundations
Read any table with precision.
1–2 weeks0/4Projection, filtering, DISTINCT and LIMIT - with NULL three-valued logic that doesn't surprise you.
Free resources
ORDER BY with ties, OFFSET pitfalls and keyset pagination at concept level.
String, date and math functions plus CASE - computing answers inside the query.
Free resources
Truncation, intervals and time zones - where analytics queries most often go quietly wrong.
- 2
Joins
Combine tables and predict the row count.
1–2 weeks0/4Matching logic, join keys and preserved sides - most interview SQL is join reasoning in disguise.
Free resources
'Customers with no orders': NOT EXISTS / LEFT JOIN … IS NULL patterns you'll use weekly.
Joining a table to itself for hierarchies and comparisons within a group.
UNION vs UNION ALL, INTERSECT and EXCEPT - stacking result sets cleanly.
- 3
Aggregation
Turn transactions into metrics.
1–2 weeks0/3Grouping rules, aggregate functions and post-aggregation filtering - the metrics engine of SQL.
Free resources
COUNT(CASE WHEN …) pivot patterns - many 'need a BI tool' requests are one query.
Subtotals and multiple groupings in a single pass.
- 4
Subqueries, CTEs & Windows
The SQL that marks you as experienced.
2 weeks0/4Scalar, IN/EXISTS and correlated subqueries - and when each beats a join.
WITH clauses for readable multi-step logic; recursive CTEs for hierarchies at concept level.
Free resources
ROW_NUMBER/RANK, PARTITION BY, running totals and moving averages - the top screening differentiator.
Free resources
The canonical window-function interview problems, solved and internalised.
- 5
Schema & Performance
Understand any database you're dropped into.
1–2 weeks0/4Primary/foreign keys, UNIQUE, NOT NULL and CHECK - integrity enforced where data lives.
Free resources
Avoiding duplication and update anomalies - and when analytics schemas denormalise on purpose.
Why one query is instant and another isn't - reading a query plan at survival level.
Free resources
Modifying data safely: affected-row checks, RETURNING and BEGIN/COMMIT discipline.
Build: Ten questions, one dataset
Load a public dataset into Postgres/SQLite and answer ten business questions; publish the queries with comments.
- 6
Prove It
Verified SQL on the resume.
1 week0/1Take the OneRoadmap SQL certification - verified, recruiter-checkable proof.
Free resources