SQL Practice

My solutions to SQL exercises, written while learning SQL for data analytics and engineering. Each file includes the problem link, the table schema, and short notes on my approach.

Progress

Source	Solved
SQLBolt	✅ 18 / 18
Select Star SQL	✅ Completed — notes
LeetCode Top SQL 50	15 / 50
DataLemur	0

Topics covered

• Filtering: WHERE, BETWEEN, IN, LIKE with % / _ wildcards
• Sorting & pagination: ORDER BY, LIMIT, OFFSET, DISTINCT
• Joins: INNER, LEFT, CROSS and self-joins; anti-join pattern (LEFT JOIN ... WHERE key IS NULL)
• NULL handling: IS NULL, COALESCE, why NULL != x is never TRUE, COUNT(*) vs COUNT(column)
• Aggregation: COUNT, SUM, AVG, MAX, GROUP BY, HAVING vs WHERE
• Conditional aggregation: AVG(CASE WHEN ... THEN 1 ELSE 0 END) for rates
• Expressions & functions: computed columns, aliases, integer vs decimal division, ROUND, DATEDIFF, CHAR_LENGTH vs LENGTH
• Query execution order: why SELECT aliases work in ORDER BY but not in WHERE
• Modifying data: INSERT, UPDATE, DELETE (verify the WHERE with a SELECT first)
• Schema: CREATE TABLE, ALTER TABLE, DROP TABLE, primary & foreign keys

LeetCode Top SQL 50

#	Problem	Difficulty	Key concept
1757	Recyclable and Low Fat Products	Easy	Filtering with AND
584	Find Customer Referee	Easy	NULL in comparisons
595	Big Countries	Easy	OR conditions
1148	Article Views I	Easy	DISTINCT, aliases
1683	Invalid Tweets	Easy	CHAR_LENGTH vs LENGTH (UTF-8)
1378	Replace Employee ID With The Unique Identifier	Easy	LEFT JOIN
1068	Product Sales Analysis I	Easy	INNER JOIN on a foreign key
1581	Customer Who Visited but Did Not Make Any Transactions	Easy	Anti-join + GROUP BY
197	Rising Temperature	Easy	Self-join, DATEDIFF
1661	Average Time of Process per Machine	Easy	Self-join on a composite key
577	Employee Bonus	Easy	LEFT JOIN + NULL filter
1280	Students and Examinations	Easy	CROSS JOIN, COUNT(column)
570	Managers with at Least 5 Direct Reports	Medium	Self-join + HAVING
1934	Confirmation Rate	Medium	Conditional aggregation
620	Not Boring Movies	Easy	Filtering, %

SQLBolt solutions

#	Lesson	Key concept
01	SELECT queries	Selecting columns
02	Queries with constraints (Pt. 1)	WHERE, BETWEEN
03	Queries with constraints (Pt. 2)	LIKE, wildcards
04	Filtering & sorting	DISTINCT, ORDER BY, LIMIT
05	Simple SELECT review	Combining clauses
06	JOINs	INNER JOIN
07	OUTER JOINs	LEFT JOIN
08	NULLs	IS NULL, anti-join
09	Expressions	Computed columns, aliases
10	Aggregates (Pt. 1)	GROUP BY
11	Aggregates (Pt. 2)	HAVING vs WHERE
12	Order of execution	Logical query order
13	Inserting rows	INSERT, primary keys
14	Updating rows	UPDATE, escaping quotes
15	Deleting rows	DELETE, orphan rows
16	Creating tables	CREATE TABLE, constraints
17	Altering tables	ALTER TABLE, defaults
18	Dropping tables	DROP TABLE

Notes

• SQLBolt runs on SQLite and LeetCode on MySQL, so some solutions include comments on where standard SQL (e.g. PostgreSQL) behaves differently.
• String literals use single quotes; columns are table-qualified in multi-table queries.