Jelaskan INNER, LEFT, RIGHT lan FULL JOIN.

Question

Accepted Answer

**JOINs** nggabungake baris saka pirang-pirang tabel adhedhasar kolom sing ana hubungane — penting banget kanggo nanyaa data relasional sing kasebar ing macem-macem tabel. Tipe join *type* nentokake baris mana sing disimpen nalika ora ana sing cocog.

## Persiapane

```sql
-- two related tables: users and their orders (orders.user_id → users.id)
SELECT users.name, orders.total
FROM users
JOIN orders ON users.id = orders.user_id;   -- combine on the relationship
```

Klausa `ON` nunjukake carane tabel-tabel kasebut ana hubungane (cocog-cocokan foreign key karo primary key). Tipe join ngontrol apa sing kedadeyan kanggo baris sing *ora* cocog.

## Papat tipe join

```text
INNER JOIN → only rows that MATCH in BOTH tables (the intersection)
             users WITH orders only — users with no orders are excluded

LEFT JOIN  → ALL rows from the LEFT table + matches from the right (NULL if no match)
             ALL users, with their orders (or NULL for users with no orders)

RIGHT JOIN → ALL rows from the RIGHT table + matches from the left (NULL if no match)
             ALL orders, with their user (mirror of LEFT)

FULL JOIN  → ALL rows from BOTH tables (matched where possible, NULL where not)
```

```sql
-- INNER — only users who have orders
SELECT u.name, o.total FROM users u INNER JOIN orders o ON u.id = o.user_id;

-- LEFT — ALL users, even those with no orders (o.total is NULL for them)
SELECT u.name, o.total FROM users u LEFT JOIN orders o ON u.id = o.user_id;
```

## Panggunaan umum: golek baris sing ORA cocog

```sql
-- LEFT JOIN + IS NULL → users who have NEVER ordered
SELECT u.name
FROM users u
LEFT JOIN orders o ON u.id = o.user_id
WHERE o.id IS NULL;          -- no matching order → o.id is NULL
```

Pola LEFT JOIN + `IS NULL` nggolek baris ing sawijining tabel sing ora ana baris sing cocog ing tabel liyane — teknik sing gampang banget lan migunani.

## Ringkesan visual

```text
INNER → A ∩ B (matches only)
LEFT  → all A + matching B
RIGHT → all B + matching A
FULL  → all A + all B
```

## Mengapa iki penting

JOINs minangka dhasar saka basis data relasional — data kanthi sengaja dipecah ing macem-macem tabel sing ana hubungane (users, orders, products), lan JOINs iku carane nggabungake data sing ana hubungane kasebut ing query, mula mangerti JOINs penting banget kanggo nyalon makok-makon kang ora gampang.

Mangerti **papat tipe join lan apa sing ditindakake marang baris sing ora cocog** minangka katrampilan utama: **INNER JOIN** (mung baris sing cocog — paling umum), **LEFT JOIN** (kabeh baris saka tabel kiwa, karo NULL kanggo sing ora cocog — penting banget nalika butuh kabeh rekaman malah ora ana sing cocog), **RIGHT JOIN** (cermine), lan **FULL JOIN** (kabeh baris saka loro-lorone).

Milih tipe join sing bener langsung nentokake asile — nggunakake INNER nalika maksud LEFT kanthi bisu-bisu ngilangi baris (bug sing gampang), mula mangerti beda-bedane penting kanggo ketepatan.

Pola **LEFT JOIN + IS NULL** (nggolek baris sing ora ana cocok ing tabel liyane — kayata user sing ora tau pakon) minangka teknik sing gampang banget lan migunani worth ngerti.

Sawisé data relasional dipecah ing macem-macem tabel lan JOINs minangka cara utama kanggo nanyaa data kasebut bebarengan, lan amarga tipe join kanthi kritis ngaruhi baris endi sing metu ing hasil, nandang JOINs — papat tipe, perilaku marang sing ora cocog, lan pola kayata anti-joins — minangka pengetahuan utama, diterapake kanthi asring kanggo SQL lan minangka konsep basis data paling penting lan paling asring ditest.