What is the difference between INNER JOIN and LEFT JOIN?

INNER JOIN returns only rows with a match in both tables; LEFT JOIN returns all rows from the left table plus matches from the right, with NULLs where there is no match.

What is the difference between WHERE and HAVING?

WHERE filters rows before grouping; HAVING filters groups after GROUP BY, typically using aggregate functions.

How do I remove duplicate rows?

Use SELECT DISTINCT to return unique rows, or GROUP BY the columns that define uniqueness.

What is the difference between DELETE and TRUNCATE?

DELETE removes rows (optionally with WHERE) and can be rolled back; TRUNCATE quickly removes all rows and usually cannot be rolled back or filtered.

What order do SQL clauses run in?

Logically: FROM, WHERE, GROUP BY, HAVING, SELECT, ORDER BY, LIMIT — which is why aliases defined in SELECT cannot be used in WHERE.

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Reference

SQL Cheat Sheet

Essential SQL syntax for everyday database work. Click any card to copy.