Why SUM(DISTINCT amount) Is Not a General Fix for JOIN Double Counting
Learn why SUM(DISTINCT amount) can discard valid payments, how SQL deduplicates rows, and when to use payment identity, EXISTS, or explicit allocation.
Category Databases
Why Your SQL SUM Is Too High: Fixing Double Counting in One-to-Many JOINs
Learn why one-to-many JOINs inflate SQL totals, why SUM(DISTINCT) can lose valid amounts, and how to fix double counting with reproducible examples.
Beyond Ten Million Rows: Evolving a Student Fitness Platform from MySQL to ClickHouse
The first scaling problem in our student fitness platform appeared in Java memory. Schools uploaded Excel files containing thousands of students’ test results. Each row passed through parsing, validation, scoring rules, and evaluation logic. During concentrated uploads, several large tasks…
Optimizing a MySQL LEFT JOIN with OR: Match Semantics Before Execution Plans
Learn how to distinguish OR matching from priority fallback, control row multiplication, and validate MySQL join rewrites with execution plans and semantic tests.