Beginner guide14 min
Learn SQL: the complete beginner's guide
A step-by-step method for moving from a business question to a reliable query, even if you've never used a database.
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SQL guides and tutorials with practical examples
Every guide includes a verified example, common mistakes to avoid and a practical next step.
Practical SQL15 min
Corrected SQL exercises: from beginner to advanced
A training method and three exercises explained to progress without transforming the correction into a simple copy and paste.
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PostgreSQL10 min
Constraints, transactions or triggers: which tool to choose?
These three tools protect data at different times. Confusing them adds complexity without improving reliability.
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Fundamentals7 min
COUNT(*) or COUNT(column)?
The difference seems small, but it can change a completeness or volume KPI.
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Discovery8 min
Excel or database: how to choose?
The right choice does not depend on a duel between tools, but on the way in which your data lives and is shared.
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Fundamentals7 min
IN, BETWEEN and LIKE: which filter to choose?
Three apparently similar operators respond to three different business intentions.
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Discovery9 min
Why does SQL exist?
SQL does not start with SELECT. It begins when multiple people and systems need to share reliable facts.
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Architecture SQL10 min
SQL beyond SELECT: create, modify, secure and automate
A database is not only used to return rows: it structures and protects the functioning of entire applications.
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Dialects SQL8 min
SQL and database: what's the difference?
SQL is a language; A database is the system that stores information and executes queries.
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Fundamentals10 min
SQL, PostgreSQL, MySQL and SQL Server: what are the differences?
SQL is the common language; PostgreSQL, MySQL and SQL Server are engines with their own dialects and tools.
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Fundamentals8 min
WHERE or HAVING: where to place your filter?
WHERE defines the calculation lines; HAVING decides which calculated groups remain visible.
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Analysis SQL9 min
CTE or subquery: how to choose?
Both structure a query; the best form is the one that makes the business steps testable.
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Fundamentals7 min
DISTINCT or GROUP BY: How to remove repeats?
Both can return a row per value, but their intent is not the same.
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joins10 min
Why does a join create duplicates?
join does not always create an error: it often reveals a poorly anticipated one-to-many relationship.
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joins8 min
INNER JOIN or LEFT JOIN: which one to choose?
The choice depends first on which lines should remain visible, not on a syntax preference.
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Data Analyst11 min
Calculate a revenue and a margin in SQL
High revenue does not guarantee high margin: the two indicators must be defined separately.
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Data Analyst11 min
CASE WHEN to SQL: create business categories
Transform a business rule into a readable column without modifying the source data.
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Analysis SQL8 min
IN or EXISTS: which writing to use?
The choice becomes simple when you distinguish membership in a list and the existence of a relationship.
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Fundamentals10 min
NULL and COALESCE to SQL: handle missing values
Handle missing values explicitly to produce reliable filters, calculations and reports.
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joins9 min
INNER JOIN to SQL: link customers and orders
Connect two tables without duplicating data with a clear and verifiable join condition.
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Data Analyst10 min
GROUP BY and HAVING: building KPIs in SQL
Transform sales lines into reliable indicators by city, category or period.
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Fundamentals7 min
SQL: understand SELECT, FROM and WHERE
Just three keywords are enough to start turning a table into a useful answer.
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