Practical guides to AI, Python, Git, SQL, Web/API, SAP/ERP and mathematical models, with explanations, code examples, sources and limitations.
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Compare RESTRICT, CASCADE and SET NULL using a small isolated example, then check the actual constraint and dependent rows before changing real data.
Use ON CONFLICT to atomically insert or update rows based on unique constraints. Specify the exact conflict target, handle duplicate input rows, and understand that excluded values replace, not accumulate.
Find the expensive step and distinguish estimates from measurements.
Transactions in PostgreSQL allow grouping SQL operations into atomic blocks using the commands BEGIN, COMMIT, and ROLLBACK. This ensures data integrity: either all changes are applied or none at all. The mechanism is based on ACID principles - atomicity, consistency, isolation, and durability.
Diagnose repeated JOIN rows by counting matching keys, checking constraints and choosing whether the result should contain orders, items or summaries.
Understand IS NULL, three-valued logic and the difference between COUNT(*) and COUNT(column).
A detailed technical breakdown of the differences between WHERE and HAVING clauses, focusing on execution order, aggregate function compatibility, and performance optimization strategies.