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Q&A
Q&AWHERE filters input rows before grouping; HAVING filters groups after aggregation.
Use WHERE to remove rows before GROUP BY, such as excluding inactive employees. Use HAVING to filter aggregate results, such as departments with COUNT(*) greater than five. Moving a condition between them can change both meaning and query work.
typescript
5 minKeep request handlers responsive by distinguishing asynchronous waiting from CPU work, bounding concurrency, and propagating stream failures.
Q&A
Q&ACOUNT(column) excludes NULL values; COUNT(*) counts input rows.
COUNT(*) counts each row in the group regardless of column values. COUNT(column) counts only rows where that expression is non-NULL. When aggregates look low, check nullability and join multiplication before changing the query.
Q&A
Q&AA one-to-many join changes row grain; multiple child joins can multiply each other.
Check the cardinality of each join. Joining one order to several lines produces multiple rows per order; joining another one-to-many table can multiply those rows again. Aggregate each relation to the intended grain before joining or use EXISTS when you only need to test presence. DISTINCT is not a reliable repair for incorrect join logic.
Q&A
Q&AA primary key gives each row a stable identity; it does not replace domain-level uniqueness constraints.
A primary key lets the database and applications refer to one row unambiguously and lets foreign keys express relationships. Add separate unique constraints for business rules such as one account per email. A generated ID alone does not prevent duplicate domain entities.
databases
10 minBuild common analytics queries while avoiding duplicate rows, incorrect aggregates, unstable pages, and cross-dialect surprises.
Q&A
Q&APromise.all aggregates promises already supplied to it. Eagerly mapping a large list can start all work at once, so downstream capacity needs a separate concurrency bound.
Promise.all aggregates promises already supplied to it. Eagerly mapping a large list can start all work at once, so downstream capacity needs a separate concurrency bound.
kubernetes
5 minUnderstand what the API stores, why controllers replace Pods, and how labels connect workloads without treating a cluster as a remote shell.