SQL Syntax Quick Reference

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Standard SQL query shape and execution order. Written order differs from meaning order: FROM → WHERE → GROUP BY → HAVING → SELECT → ORDER BY.

Query Shape

SELECT [DISTINCT] A1, A2, ...        -- projection (keeps duplicates unless DISTINCT)
FROM r1 [JOIN TYPE] r2 ON pred       -- meaning order: FROM → WHERE → GROUP BY → HAVING → SELECT
WHERE predicate                      -- unknown rows dropped like false
GROUP BY A1 [, A2]
HAVING group_predicate               -- filters groups after grouping
ORDER BY A [ASC|DESC] [NULLS FIRST|LAST]

Joins

Form	Behavior
`JOIN ... ON pred`	Arbitrary predicate; filters before outer-padding
`LEFT/RIGHT/FULL OUTER JOIN`	Pads unmatched rows with NULLs
`NATURAL JOIN`	Equates ALL shared attributes — risky if schema gains columns
`JOIN ... USING (A)`	Equates only named attributes

Aggregation

Construct	Notes
`COUNT(*) / COUNT(col) / COUNT(DISTINCT col)`	COUNT(col) ignores NULLs
`SUM, AVG, MIN, MAX`	Ignore NULLs (AVG = SUM/COUNT non-null)
`GROUP BY` + `HAVING`	HAVING filters groups; WHERE filters rows before grouping

Nulls & Three-Valued Logic

NULL = NULL is UNKNOWN, not TRUE. Use IS NULL / IS NOT NULL. WHERE keeps only TRUE rows; UNKNOWN is dropped like FALSE.

Window Functions

SELECT rank() OVER (PARTITION BY dept ORDER BY salary DESC) AS rnk
FROM instructor;

PARTITION BY groups; ORDER BY inside OVER orders within each partition.