How to compare this month with last month and with the same month last year in SQL: LAG for month-over-month, a safe date join for year-over-year, year-to-date totals, and the missing-month trap.
Delphi exception handling explained: try..except vs try..finally, catching specific exceptions, raise and re-raise, custom exceptions, FireDAC errors.
Temp table, table variable or CTE? How each one really works in SQL Server, why table variables can get a 1-row estimate, why a CTE can run twice, and a simple rule for choosing.
OOP in Delphi for beginners: classes, objects, properties, constructors, try..finally with Free, inheritance, virtual methods and TObjectList.
Use parameters in Delphi SQL queries with FireDAC and ADO: ParamByName, data types, NULLs, dates, LIKE, INSERT, and how they stop SQL injection.
Our review of Designing Data-Intensive Applications, 2nd edition, by Martin Kleppmann and Chris Riccomini: what it covers, who it is for, and whether to upgrade.
How to turn rows into columns in SQL: conditional aggregation that works everywhere, the SQL Server PIVOT operator, dynamic pivot, and two ways to unpivot (without losing NULLs).
Why a query suddenly gets slow after a data load: how SQL Server statistics work, how to compare estimated and actual rows, check when stats were last updated, and keep them fresh.
The same analysis in SQL and pandas side by side, a cheat sheet that maps one to the other, where each is stronger, and how analysts use both together.
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.
