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Clear filtersINNER JOIN returns only rows that have a match in both tables, while LEFT JOIN returns every row from the left table and fills the right table's columns with NULL when there is no match.
WHERE filters individual rows before grouping, while HAVING filters groups after GROUP BY, so only HAVING can use aggregate functions like COUNT or SUM.
DELETE removes selected rows and can be rolled back, TRUNCATE quickly removes all rows and keeps the table structure, and DROP removes the entire table including its structure.
UNION combines two result sets and removes duplicate rows, while UNION ALL combines them and keeps every row, which makes it faster.
Use a subquery that takes the MAX salary below the overall MAX, or use DENSE_RANK() and pick rank 2, which also handles ties and generalises to the Nth highest.
Find duplicates with GROUP BY and HAVING COUNT(*) > 1, then delete them by numbering rows within each duplicate group with ROW_NUMBER() and removing every row after the first.