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Prove your SQL skills

Most people who list SQL can write a SELECT. Fewer can explain why a join duplicated rows or why a query got slow. Lemma records that difference through live query sessions and an adaptive assessment.

How SQL is assessed

Preparing for an interview? SQL interview questions, junior to senior

What SQL competence looks like

A competent SQL practitioner starts by understanding the grain of each table. Before writing a join they ask what one row represents, so they notice when a join will fan out and inflate a total. They handle NULLs deliberately and test a query against cases where the answer is known.

They write queries other people can follow, using CTEs to name intermediate steps and window functions instead of tangled self-joins. When a query is slow, they read the execution plan, check which indexes are used and change the query or the index rather than guessing. They know when a transaction or a constraint should protect the data.

SQL session
Live · 41:07

Writing queries on an unfamiliar schema

60 min · Learning · with Noah B.

monthly_revenue.sql Kenji M. is sharing
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Noah B. rated the session

Practitioner · after the call

Clear reasoning on joins and table grain. Talked through trade-offs first.

Added to SQLAdded to record

Joins and table grain

Inner, outer and anti joins, and spotting fan-out before it silently inflates results.

Aggregation and grouping

GROUP BY, HAVING and conditional aggregation, with correct handling of NULLs and duplicates.

Window functions

Ranking, running totals and comparisons across rows without self-joins or subquery sprawl.

CTEs and readability

Breaking a long query into named steps that reviewers can check one at a time.

Indexes and query plans

Reading execution plans, choosing indexes and rewriting queries that scan more than they need.

Schema and integrity

Keys, constraints, normalisation trade-offs and transactions that keep data consistent.

What counts as evidence of SQL skills?

In SQL, a polished result can hide a shaky method. Lemma records the evidence behind the method: sessions where someone followed your reasoning, an adaptive test and the repositories you link, notebooks included. Each source is listed below. See the evidence methodology for how each source is weighed.

Skills Passport
Signed
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SQL

Lucia F. · SCI recomputed 9 Oct

Evidence · 4 entries

  1. Writing queries on an unfamiliar schema

    Live session · 9 Oct

    Rated 5.0

  2. Adaptive SQL scenario questions

    Adaptive assessment · 2 Oct

    Level 7.2

  3. Queries and migrations on GitHub

    GitHub connector · 24 Sep

    Verified

  4. Window function practice

    AI tutor practice · 11 Sep

    Practice
Signature valid · checked today Share link
Live session

Writing queries on an unfamiliar schema

A practitioner gives you a schema and a business question. You explore the tables, write the query and explain your assumptions about grain and NULLs while they watch and rate the session.

Adaptive assessment

Adaptive SQL scenario questions

Multiple-choice questions test joins, grouping and window functions. Some show a query, such as a join that double counts, and ask you to pick the correct diagnosis. Harder questions count for more.

GitHub connector

Queries and migrations on GitHub

Connect a repository with analytics models, migrations or a reporting project. Lemma checks that the repository exists and that your account has access to it.

AI tutor practice

Window function practice

AI tutor drills on ranking, partitions and frames build speed before a live session. They count as practice, adding only a small, capped amount to your score.

How is an SQL skills test run on Lemma?

The SQL test asks 15 to 25 questions and moves up or down a level with each answer, focusing on judgement calls such as leakage, skew and bad joins. It feeds your SQL SCI together with sessions and linked work, and the score weighs how fresh that evidence is.

How the SCI score is calculatedHow adaptive skills assessments workPractise SQL in live skill swap sessionsWhich evidence types countWhat your Skills Passport shows
SQL assessment
0:42
Question 11Stops when your level holds
Difficulty7/10

Revenue doubles after joining orders to shipments. What is the most likely cause?

  1. AOrders with two shipments repeat each row
  2. BGROUP BY is missing a column
  3. CLEFT JOIN drops unmatched rows
  4. DSUM ignores NULL values
Locks on submit

  1. Practise with targeted drills

    The AI tutor sets SQL exercises that adapt to you, from joins to window frames, and remembers what you covered. Drills prepare you; they contribute only a small, capped amount to your practice score.

  2. Query live with a practitioner

    In one-to-one sessions you answer real questions against a schema, explain your logic and tune a slow query. The practitioner's rating is recorded as SQL evidence after the session.

  3. Pass the SQL assessment

    Scenario questions get harder or easier based on your answers. Some show a query with a wrong result and ask you to spot why. You cannot revisit answers, and failed attempts slightly lower your proof score.

SQL skills: common questions

How do I prove my SQL skills for a data analyst job?

Show that your queries return the right answer and that you can explain why. Write queries live against an unfamiliar schema with a practitioner, pass the adaptive SQL assessment, and connect a repository with real queries or models. Your Passport then shows analysis-ready SQL rather than a line on a CV.

What does an SQL skills test usually check?

Good SQL tests check joins and their side effects, aggregation, NULL handling, window functions and readability. Stronger ones also ask about indexes and query plans. Lemma's assessment covers these with realistic scenarios, including queries where you pick the correct diagnosis.

Does it matter which SQL dialect I use?

The core ideas transfer across PostgreSQL, MySQL, SQL Server and warehouse dialects, and that is what the assessment focuses on. In live sessions you can work in the dialect you know best. Mention dialect-specific experience in your sessions so the practitioner's rating reflects it.

How do interviewers tell basic SQL from advanced SQL?

They look at how you handle fan-out in joins, whether you reach for window functions, how you treat NULLs and whether you can read an execution plan. Advanced practitioners also explain trade-offs between rewriting a query and adding an index. Live sessions on Lemma probe exactly these points.

Skills related to SQL

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Evidence that your pipelines are reliable, testable and recoverable, reviewed by practitioners in live sessions.

Evidence for data engineering

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Show how you write, debug and structure Python, not just that you once finished a course.

Evidence for Python

Start your SQL evidence record.

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