SQL

EXISTS/NOT EXISTS
UNION (discards duplicates)/UNION ALL (preserves duplicates)
Normalization rules
Subquery vs. correlated query - correlated subqueries are performed once for every row in outer query

COUNT(*) includes NULLs
COUNT(DISTINCT) ignores NULLs
JOINS: INNER, LEFT OUTER, RIGHT OUTER, FULL (both LEFT and RIGHT OUTER)
SELECTing NULL columns: NULL is concatenated with any string it results in NULL
VIEW cannot have an ORDER BY clause