Cohort and Retention Analysis in SQL (Step by Step)
Group customers by the month they first bought and track how many come back. Build a retention triangle in SQL Server and MySQL, step by step.
Funnel Analysis in SQL: Find Where Users Drop Off (Step by Step)
Build a conversion funnel in SQL: count users per step, enforce step order, calculate drop-off with LAG, add a conversion window and compare mobile vs desktop.
Many-to-Many Relationships in SQL: Junction Tables Explained Simply
How to store many-to-many relationships in SQL with a junction table: composite keys, foreign keys, indexes, queries and the comma-list mistake.
Delphi Procedures, Functions and Units Explained Simply
Split Delphi code into procedures, functions and units: value, const, var and out parameters, default parameters, overloads and the unit structure.
How to Connect Delphi to SQL Server with FireDAC (Step by Step)
Connect Delphi to SQL Server with FireDAC: TFDConnection settings, Windows vs SQL login, connecting in code, the ADO alternative and common errors.
Learning SQL (3rd Edition) Review: The Best SQL Book for Serious Beginners?
An honest review of Learning SQL, 3rd edition, by Alan Beaulieu: what it covers, who it suits, its strengths and weaknesses, and how it compares to other SQL books.
EXISTS vs IN vs JOIN in SQL: Which One Should You Use?
The same question answered with EXISTS, IN and JOIN: when each one is right, why JOIN can duplicate rows, the NOT IN NULL trap, and what really matters for speed.
SQL for Data Analysis: A Step-by-Step Roadmap
What to learn, in what order, to use SQL for real data analysis: 9 stages with free tutorials, an 8-week study plan and a first portfolio project.
Aging Report in SQL Server: Group Unpaid Invoices into 0-30, 31-60, 61-90 and 90+ Days
Build an accounts receivable aging report in SQL Server step by step: open amount per invoice, days overdue, aging buckets per customer, totals, and common mistakes.
Basic Statistics in SQL: Average, Median, Percentiles and Standard Deviation
Calculate averages, medians, percentiles, mode and standard deviation in SQL Server and MySQL, find outliers, and learn which number to report.