Chuleta de joins de SQL
Una referencia de joins de SQL con búsqueda e imprimible: inner, left, right, full, anti y semi joins, self joins, ON vs WHERE y 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
Ninguna entrada coincide con “:q”.
Acerca de Chuleta de joins de SQL
Esta chuleta de joins de SQL hace una cosa a fondo: explica todas las formas de combinar tablas. Ocho secciones cubren los tipos de join básicos, los anti-joins para filas sin coincidencia, los semi-joins para filas que sí tienen coincidencia, los self joins, los joins cruzados y multitabla, la clásica trampa de ON frente a WHERE, USING, NATURAL y las operaciones de conjuntos, y los joins LATERAL con notas de rendimiento.
Los joins son donde los resultados de SQL fallan silenciosamente: un LEFT JOIN filtrado en la cláusula WHERE se convierte en silencio en un INNER JOIN, y NOT IN con un NULL no devuelve nada. Esas trampas tienen sus propias filas aquí, junto al patrón correcto que usar en su lugar.
Como cada chuleta de este grupo, es gratuita y se ejecuta en el cliente: filtra filas en vivo con el cuadro de búsqueda, salta entre secciones con el índice fijo, copia cualquier sentencia con un clic e imprime la página como referencia en tu escritorio.
Cómo usar Chuleta de joins de SQL
- Abre la hoja y revisa las secciones, desde Tipos de join básicos hasta LATERAL y rendimiento.
- Busca una palabra clave como LEFT JOIN, EXISTS o NOT IN para filtrar todas las filas en vivo.
- Lee la sección ON vs WHERE antes de depurar un LEFT JOIN que devuelve muy pocas filas.
- Haz clic en una sentencia o en su icono de copiar para copiar el SQL a tu portapapeles.
- Usa Imprimir para obtener una copia en papel de la referencia completa de joins.
Preguntas frecuentes
Inner, left, right y full outer joins, además de anti-joins, semi-joins, self joins, joins cruzados, joins multitabla, joins USING y NATURAL, la familia UNION de operaciones de conjuntos, y joins LATERAL.
Una condición en ON filtra la tabla unida antes de que el outer join conserve las filas sin coincidencia; la misma condición en WHERE se ejecuta después y descarta las filas NULL, convirtiendo el LEFT JOIN en un INNER JOIN. La hoja tiene una sección dedicada exactamente a esto.
La sección de anti-joins muestra los patrones LEFT JOIN … WHERE right.id IS NULL y NOT EXISTS, y explica por qué NOT IN se comporta mal cuando la subconsulta puede devolver NULL.
Sí. La sintaxis es SQL estándar y funciona en MySQL, PostgreSQL, SQL Server y SQLite, con las pocas notas específicas de cada motor señaladas en la propia fila.
Sí, es completamente gratis y se ejecuta en tu navegador sin inicio de sesión.
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