Helaian Rujukan SQL Joins
Rujukan sql joins yang boleh dicari dan dicetak: inner, left, right, full, anti dan semi join, self join, ON vs WHERE dan LATERAL. Percuma.
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
Tiada entri sepadan dengan “:q”.
Mengenai Helaian Rujukan SQL Joins
Helaian rujukan sql joins ini melakukan satu perkara secara menyeluruh: ia menerangkan setiap cara untuk menggabungkan jadual. Lapan bahagian merangkumi jenis join asas, anti-join untuk baris tanpa padanan, semi-join untuk baris yang mempunyai padanan, self join, cross dan multi-table join, perangkap klasik ON berbanding WHERE, USING, NATURAL dan operasi set, serta LATERAL join berserta nota prestasi.
Join adalah tempat hasil SQL secara senyap tersasar: LEFT JOIN yang ditapis dalam klausa WHERE secara senyap bertukar menjadi INNER JOIN, dan NOT IN dengan NULL memulangkan tiada apa-apa. Perangkap itu mendapat barisnya sendiri di sini, bersebelahan corak yang betul untuk digunakan sebagai gantinya.
Seperti setiap helaian rujukan dalam kumpulan ini, ia percuma dan di sisi klien: tapis baris secara langsung dengan kotak carian, lompat antara bahagian dengan senarai kandungan yang melekat, salin sebarang penyataan dengan satu klik dan cetak halaman untuk rujukan di meja anda.
Cara menggunakan Helaian Rujukan SQL Joins
- Buka helaian itu dan semak bahagiannya, dari Basic join types hingga LATERAL & performance.
- Cari kata kunci seperti LEFT JOIN, EXISTS atau NOT IN untuk menapis setiap baris secara langsung.
- Baca bahagian ON vs WHERE sebelum mengesan pepijat pada LEFT JOIN yang memulangkan terlalu sedikit baris.
- Klik sesuatu penyataan atau ikon salinnya untuk menyalin SQL itu ke papan keratan anda.
- Gunakan Print untuk salinan bercetak bagi keseluruhan rujukan joins.
Soalan lazim
Inner, left, right dan full outer join, ditambah anti-join, semi-join, self join, cross join, multi-table join, USING dan NATURAL join, keluarga operasi set UNION, serta LATERAL join.
Syarat dalam ON menapis jadual yang digabungkan sebelum outer join mengekalkan baris yang tidak sepadan; syarat yang sama dalam WHERE berjalan selepas itu dan membuang baris NULL, menukar LEFT JOIN menjadi INNER JOIN. Helaian ini mempunyai satu bahagian khusus mengenai perkara itu.
Bahagian anti-join menunjukkan corak LEFT JOIN … WHERE right.id IS NULL dan NOT EXISTS, serta menerangkan mengapa NOT IN berkelakuan tidak diingini apabila subquery boleh memulangkan NULL.
Ya. Sintaksnya adalah SQL standard dan berfungsi dalam MySQL, PostgreSQL, SQL Server dan SQLite, dengan beberapa nota khusus enjin dinyatakan pada barisnya sendiri.
Ya, ia sepenuhnya percuma dan berjalan dalam pelayar anda tanpa log masuk.
Carian popular
sql joins cheat sheet
sql join types
inner join vs left join
sql anti join
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on vs where in joins
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