SQL Concepts Every Developer Should Know: What, Why, When, Where, and How

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https://www.debug.school/rakeshdevcotocus_468/sql-concepts-every-developer-should-know-what-why-when-where-and-how-44d8

Write a comprehensive, interview-focused Markdown blog titled “SQL Concepts Every Developer Should Know: What, Why, When, Where, and How” using simple English and PostgreSQL syntax. Begin with an introduction, a quick-overview table, and a shared employees-and-departments database with CREATE TABLE statements, INSERT commands, and sample records. Cover table creation, keys, constraints, normalization, SELECT, aliases, DISTINCT, filtering operators, sorting, pagination, NULL handling, aggregate functions, GROUP BY, HAVING, CASE, all major joins, scalar and correlated subqueries, EXISTS, set operations, CTEs, window functions, INSERT, UPDATE, DELETE, ALTER, TRUNCATE, DROP, views, indexes, EXPLAIN, transactions, savepoints, ACID, isolation levels, permissions, and parameterized queries. Give each concept a descriptive heading followed by What, Why, When and Where, How, a practical SQL example, its expected output, and an interview question with a clear answer. Include solved problems for second-highest distinct salary, top N employees per department, duplicate detection, running totals, and previous-row comparisons, explaining ties and NULL-related edge cases. Keep examples consistent, state whether sections run independently, demonstrate mutations safely, and distinguish expected results from measured execution plans. Explain logical SELECT processing order, summarize common interview mistakes in a comparison table, cite official documentation, and finish with colorful SQL concept-map and query-processing diagrams, providing Mermaid code and readable diagram images suitable for the bottom of the blog.