Cheat sheet JOIN SQL
Una referenza consultabile e stampabile sui join SQL: inner, left, right, full, anti e semi join, self join, ON vs WHERE e LATERAL. Gratis.
Basic join types
9SELECT * FROM a INNER JOIN b ON a.id = b.a_id;
Only rows with a match in BOTH tables
SELECT * FROM a LEFT JOIN b ON a.id = b.a_id;
All left rows, NULLs where no match
SELECT * FROM a RIGHT JOIN b ON a.id = b.a_id;
All right rows, NULLs where no match
SELECT * FROM a FULL OUTER JOIN b ON a.id = b.a_id;
All rows from both, NULLs if unmatched
JOIN = INNER JOIN
Plain JOIN defaults to INNER
LEFT OUTER JOIN = LEFT JOIN
The OUTER keyword is optional
LEFT JOIN ... UNION ... RIGHT JOIN ...
Emulate FULL JOIN in MySQL (none native)
ON a.id = b.a_id AND a.type = b.type
Compound (multi-column) join condition
ON a.qty BETWEEN p.min_qty AND p.max_qty
Non-equi join: match on a range
Anti-joins (no match)
8LEFT JOIN b ON a.id = b.a_id WHERE b.a_id IS NULL
Left rows with NO match on the right
WHERE NOT EXISTS (SELECT 1 FROM b WHERE b.a_id = a.id)
Anti-join; safe with NULLs
WHERE a.id NOT IN (SELECT a_id FROM b)
One NULL in the list = empty result!
WHERE a.id NOT IN (SELECT a_id FROM b WHERE a_id IS NOT NULL)
NOT IN made NULL-safe
RIGHT JOIN a ... WHERE a.id IS NULL
Right rows with no match on the left
SELECT id FROM a EXCEPT SELECT a_id FROM b;
Set difference (Postgres, MySQL 8.0.31+)
users without orders = LEFT JOIN + IS NULL
The classic anti-join use case
Prefer NOT EXISTS over NOT IN
Same plan, none of the NULL traps
Semi-joins (has a match)
8WHERE EXISTS (SELECT 1 FROM b WHERE b.a_id = a.id)
Left rows having a match; never duplicates
WHERE a.id IN (SELECT a_id FROM b)
Same intent; fine for short value lists
JOIN b ... GROUP BY a.id
JOIN duplicates 1-to-many; dedupe needed
SELECT DISTINCT a.* FROM a JOIN b ON ...
Semi-join via JOIN; EXISTS is cleaner
EXISTS stops at the first match
Often faster than joining everything
EXISTS (SELECT 1 ...) vs (SELECT * ...)
Selected columns are ignored; 1 is idiomatic
IN: uncorrelated / EXISTS: correlated
Optimizers usually rewrite them the same
users WITH orders = EXISTS subquery
The classic semi-join use case
Self joins
8FROM emp e JOIN emp m ON e.manager_id = m.id
Each employee paired with their manager
FROM emp e LEFT JOIN emp m ON e.manager_id = m.id
Keeps top-level rows (manager is NULL)
SELECT e.name, m.name AS manager FROM ...
Aliases are REQUIRED to tell copies apart
ON a.id < b.id
Unique pairs: no self-match, no mirrors
ON a.id <> b.id
All ordered pairs except self-matches
ON b.day = a.day + INTERVAL 1 DAY
Adjacent rows (gaps and islands)
ON a.city = b.city AND a.id <> b.id
Other rows sharing the same value
WITH RECURSIVE tree AS (... UNION ALL ...)
Whole hierarchy: recursive CTE, not a join
Cross & multi-table joins
8SELECT * FROM sizes CROSS JOIN colors;
Every combination: m x n rows, no ON
FROM sizes, colors
Implicit cross join; avoid this syntax
FROM users u JOIN orders o ON ... JOIN items i ON ...
Chain joins; each needs its own ON
JOIN dates d CROSS JOIN users u LEFT JOIN ...
Calendar x users grid, then fill data
u LEFT JOIN o ON ... LEFT JOIN i ON i.o_id = o.id
Keep LEFT down the chain to keep all users
u LEFT JOIN o ... INNER JOIN i ON i.o_id = o.id
INNER after LEFT drops the NULL rows!
FROM a JOIN (b LEFT JOIN c ON ...) ON ...
Parentheses control join grouping
Written order is logical, not execution order
The optimizer reorders inner joins freely
ON vs WHERE (the gotcha)
8LEFT JOIN b ON a.id = b.a_id AND b.status = 1
Filters RIGHT side; all left rows kept
LEFT JOIN b ON a.id = b.a_id WHERE b.status = 1
NULL fails the test: LEFT becomes INNER!
WHERE b.a_id IS NULL OR b.status = 1
Keep unmatched rows despite a WHERE filter
LEFT JOIN b ON ... AND a.type = 'x'
Left-side filter in ON does NOT drop rows
WHERE a.type = 'x'
Filter the LEFT table in WHERE instead
INNER JOIN: ON and WHERE are equivalent
The trap only exists for outer joins
COALESCE(b.total, 0)
Replace outer-join NULLs with a default
COUNT(b.id) vs COUNT(*)
COUNT(col) skips NULLs: 0 for no match
USING, NATURAL & set ops
9JOIN b USING (user_id)
Equi-join on a same-named column
USING outputs ONE shared column
ON keeps both a.user_id and b.user_id
NATURAL JOIN b
Auto-joins ALL same-named columns; fragile
Avoid NATURAL JOIN in production
A new column silently changes the join
SELECT ... UNION SELECT ...
Stack rows vertically, duplicates removed
UNION ALL
Keep duplicates; much faster than UNION
JOIN adds columns, UNION adds rows
Widen vs lengthen the result set
INTERSECT
Rows present in both result sets
UNION needs same column count and types
Names come from the first SELECT
LATERAL & performance
9CROSS JOIN LATERAL (SELECT ... WHERE b.a_id = a.id LIMIT 3) t
Per-row subquery: top-N per group
LEFT JOIN LATERAL (...) t ON TRUE
Keep left rows whose subquery is empty
CROSS APPLY / OUTER APPLY
SQL Server spelling of LATERAL joins
LATERAL sees columns of preceding tables
A plain subquery in FROM cannot
CREATE INDEX idx ON orders (user_id);
Index the FK side of every frequent join
EXPLAIN SELECT ...;
Check join order, type, and index usage
Filter early: join the smallest set first
Less rows carried through each join
ON DATE(a.created_at) = b.day
Function on a column kills index use
Join on same-type, same-collation columns
Implicit casts also disable indexes
Nessuna voce corrisponde a “:q”.
Informazioni su Cheat sheet JOIN SQL
Questo cheat sheet sui join SQL fa una cosa in modo approfondito: spiega ogni modo di combinare le tabelle. Otto sezioni coprono i tipi di join di base, gli anti-join per righe senza corrispondenza, i semi-join per righe che hanno una corrispondenza, i self join, i join incrociati e multi-tabella, la classica trappola ON contro WHERE, USING, NATURAL e le operazioni insiemistiche, e i join LATERAL con note sulle prestazioni.
I join sono dove i risultati SQL vanno silenziosamente storti: un LEFT JOIN filtrato nella clausola WHERE diventa silenziosamente un INNER JOIN, e NOT IN con un NULL non restituisce nulla. Queste trappole hanno le proprie righe qui, accanto al pattern corretto da usare al loro posto.
Come ogni cheat sheet di questo gruppo è gratuito e lato client: filtra le righe dal vivo con la casella di ricerca, salta tra le sezioni con l'indice fisso, copia qualsiasi istruzione con un clic e stampa la pagina come riferimento sulla scrivania.
Come usare Cheat sheet JOIN SQL
- Apri il foglio e rivedi le sezioni, dai Tipi di join di base fino a LATERAL e prestazioni.
- Cerca una parola chiave come LEFT JOIN, EXISTS o NOT IN per filtrare ogni riga dal vivo.
- Leggi la sezione ON contro WHERE prima di fare debug di un LEFT JOIN che restituisce troppe poche righe.
- Fai clic su un'istruzione o sulla sua icona di copia per copiare l'SQL negli appunti.
- Usa Stampa per una copia cartacea dell'intero riferimento sui join.
Domande frequenti
Inner, left, right e full outer join, più anti-join, semi-join, self join, cross join, join multi-tabella, join USING e NATURAL, la famiglia UNION di operazioni insiemistiche, e i join LATERAL.
Una condizione in ON filtra la tabella unita prima che il join esterno mantenga le righe senza corrispondenza; la stessa condizione in WHERE viene eseguita dopo e scarta le righe NULL, trasformando il LEFT JOIN in un INNER JOIN. Il foglio ha una sezione esattamente su questo.
La sezione sugli anti-join mostra i pattern LEFT JOIN … WHERE right.id IS NULL e NOT EXISTS, e spiega perché NOT IN si comporta male quando la sottoquery può restituire NULL.
Sì. La sintassi è SQL standard e funziona in MySQL, PostgreSQL, SQL Server e SQLite, con le poche note specifiche del motore segnalate direttamente sulla riga.
Sì, è completamente gratuito e funziona nel tuo browser senza accesso.
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