SQL flashcards, generated for you.

Structured Query Language. Used to query, insert, update, and delete data in relational databases.

SELECT → FROM → WHERE → GROUP BY → HAVING → ORDER BY → LIMIT

WHERE filters individual rows before aggregation occurs. Use HAVING after GROUP BY to filter aggregated results instead.

INNER JOIN returns only rows with matches in both tables. LEFT JOIN returns all rows from the left table plus matching rows from the right table (NULLs for non-matches).

NULL represents missing or unknown data. Test with IS NULL or IS NOT NULL; using = NULL always returns false because NULL is not equal to anything, including itself.

DISTINCT removes duplicate rows from results. GROUP BY organizes rows into groups and is required when using aggregate functions; it can also remove duplicates but is slower and designed for aggregation.

Only the grouped columns and any columns with aggregate functions should be in SELECT. Non-aggregated, non-grouped columns produce undefined behavior in most SQL databases.

UNION combines results from multiple queries and removes duplicate rows (slower). UNION ALL keeps all rows including duplicates (faster). Both require matching column counts and types.

If the join condition matches multiple rows in one or both tables, the result contains all combinations (Cartesian product of matches). Always verify join cardinality with sample data.

A subquery (nested SELECT) returns data to another query. Use subqueries for: filtering with IN/NOT IN, single aggregate values in WHERE, or when a JOIN would create unwanted duplicates. JOINs are usually faster.