SQL — Junior · Resources

1SELECT fluency

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Junior SQL is about reading and writing basic queries without hesitation. Drill until the syntax is automatic.

The core query

SELECT / FROM / WHERE / ORDER BY / LIMIT.

SQLBolt — lessons 1–6.

Column expressions & arithmetic; AS aliases.

Mode — SELECT.

Sorting by multiple columns, ASC/DESC, and NULLS LAST.

PostgreSQL — ORDER BY.

2Filtering & operators

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Precise WHERE clauses are where most beginner bugs live.

WHERE mastery

AND / OR precedence and parentheses.

SQLBolt — constraints.

IN, BETWEEN, LIKE (wildcards %/_).

W3Schools — LIKE.

CASE WHEN for conditional expressions.

Mode — CASE.

3DISTINCT, aliases & NULL

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NULL behaviour is the single most common junior trap.

Handle the edges

IS NULL vs = NULL; why NULL = NULL is not true.

W3Schools — NULL.

COALESCE / NULLIF; NULLs in aggregates (COUNT(col) skips them).

PostgreSQL — COALESCE.

DISTINCT and de-duplicating result rows.

W3Schools — DISTINCT.

4Aggregates & grouping

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The first "real" SQL skill: summarising rows into numbers.

Summarise data

COUNT / SUM / AVG / MIN / MAX.

SQLBolt — aggregates.

GROUP BY — and why you can't select a non-grouped column.

SQLBolt — GROUP BY.

HAVING vs WHERE — filter groups vs rows.

W3Schools — HAVING.

COUNT(*) vs COUNT(col) vs COUNT(DISTINCT col).

PostgreSQL — aggregates.

5Joins & keys

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Data lives across tables. Junior interviews always check you can join two tables correctly.

Connect tables

INNER vs LEFT join; when rows disappear or become NULL.

SQLBolt — JOINs.

Primary key vs foreign key; what a relationship means.

W3Schools — Foreign key.

Constraints: NOT NULL, UNIQUE, CHECK, DEFAULT.

W3Schools — Constraints.

Joining more than two tables; aliasing tables for readability.

SQLBolt — Joins.

6Set operations

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Combining result sets — small but frequently asked.

Combine results

UNION vs UNION ALL (dedupe vs not, and the cost).

W3Schools — UNION.

INTERSECT / EXCEPT at a glance.

PostgreSQL — set ops.

7String & date functions

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The tools behind "revenue per month" and text-cleanup questions.

Everyday functions

CONCAT, SUBSTRING, UPPER/LOWER, TRIM, REPLACE.

Mode — String functions.

NOW(), DATE_TRUNC, EXTRACT, date arithmetic.

PostgreSQL — date/time.

8Drill it

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Repetition builds the muscle memory that makes live SQL painless.

Practice

Finish all SQLBolt interactive lessons.

SQLBolt.

Start the LeetCode SQL 50 easy problems.

LeetCode — SQL 50.