Lembar Contekan SQL Joins
Referensi SQL joins yang bisa dicari dan dicetak: inner, left, right, full, anti, dan semi join, self join, ON vs WHERE, dan 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
Tidak ada entri yang cocok dengan “:q”.
Tentang Lembar Contekan SQL Joins
Contekan SQL joins ini melakukan satu hal secara menyeluruh: menjelaskan setiap cara menggabungkan tabel. Delapan bagian mencakup jenis join dasar, anti-join untuk baris tanpa kecocokan, semi-join untuk baris yang memang memiliki kecocokan, self join, cross join dan join multi-tabel, jebakan klasik ON versus WHERE, USING, NATURAL dan operasi set, serta LATERAL join dengan catatan performa.
Join adalah tempat hasil SQL diam-diam menjadi salah: LEFT JOIN yang difilter di klausa WHERE diam-diam berubah menjadi INNER JOIN, dan NOT IN dengan NULL tidak mengembalikan apa pun. Jebakan tersebut mendapat barisnya sendiri di sini, berdampingan dengan pola yang benar untuk digunakan sebagai gantinya.
Seperti setiap contekan dalam grup ini, gratis dan berjalan di sisi klien: filter baris secara langsung dengan kotak pencarian, berpindah antar bagian dengan daftar isi yang menempel, salin statement apa pun dengan satu klik dan cetak halaman untuk referensi di meja kerja Anda.
Cara menggunakan Lembar Contekan SQL Joins
- Buka contekan dan tinjau bagian-bagiannya, dari Basic join types hingga LATERAL & performance.
- Cari kata kunci seperti LEFT JOIN, EXISTS atau NOT IN untuk memfilter setiap baris secara langsung.
- Baca bagian ON vs WHERE sebelum men-debug LEFT JOIN yang mengembalikan terlalu sedikit baris.
- Klik statement atau ikon salinnya untuk menyalin SQL ke clipboard Anda.
- Gunakan Cetak untuk salinan kertas dari referensi join lengkap.
Pertanyaan yang sering diajukan
Inner, left, right dan full outer join, ditambah anti-join, semi-join, self join, cross join, join multi-tabel, join USING dan NATURAL, keluarga operasi set UNION, serta LATERAL join.
Kondisi dalam ON menyaring tabel yang di-join sebelum outer join mempertahankan baris yang tidak cocok; kondisi yang sama dalam WHERE berjalan setelahnya dan membuang baris NULL, mengubah LEFT JOIN menjadi INNER JOIN. Contekan ini memiliki bagian khusus tentang hal itu.
Bagian anti-join menunjukkan pola LEFT JOIN … WHERE right.id IS NULL dan NOT EXISTS, dan menjelaskan mengapa NOT IN berperilaku salah saat subquery bisa mengembalikan NULL.
Ya. Sintaksnya adalah SQL standar dan berfungsi di MySQL, PostgreSQL, SQL Server dan SQLite, dengan beberapa catatan khusus mesin disebutkan pada barisnya sendiri.
Ya, sepenuhnya gratis dan berjalan di browser Anda tanpa login.
Pencarian populer
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