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Joins, window functions, aggregation, and the analytical queries asked in a shared editor.
3 questions in Databases & SQL.
SQL join duplicates cause double counting when two one-to-many joins multiply rows before aggregation. Aggregate each child table to one row per parent first, then join those results back to the parent. Use this sql queries answer to show the decision, trade-off, and evidence rather than a memorised definition.
Use NOT EXISTS for customers with no recent order because it expresses an anti-join and avoids NULL traps. NOT IN can return no rows if the subquery contains NULL, and LEFT JOIN becomes wrong if filters are placed in WHERE. Use this SQL answer to show the decision, trade-off, and evidence rather than a memorised definition. It also connects null semantics to the point an interviewer is testing.
To get the second highest salary per department in SQL, rank employees inside each department with DENSE_RANK and filter rank two. DENSE_RANK handles ties where ROW_NUMBER or a global MAX gives the wrong result. Use this window functions answer to show the decision, trade-off, and evidence rather than a memorised definition. It also connects sql queries to the point an interviewer is testing.