Terangna INNER, LEFT, lan RIGHT JOIN ing MySQL nganggo tuladha.

Question

IT Interview Practice · Accepted Answer

JOIN nggabungake baris saka rong tabel kanthi nyocokaké sawijining kahanan, biasané foreign key. *Tipe* join nemtokake apa sing kedadéan marang baris sing **ora cocog** ing sisih liyané.

## Telu sing kokbutuhaké

- **INNER JOIN** — mung njaga baris sing cocog ing loro-loroné sisih.
- **LEFT JOIN** — njaga saben baris saka tabel kiwa; kolom tengen sing ora cocog bali dadi `NULL`.
- **RIGHT JOIN** — kosok baliné: njaga saben baris tengen. Arang dienggo; wong luwih milih mbalik tabelé lan nulis LEFT.

```sql
-- customers(id, name)   orders(id, customer_id, total)

-- INNER: only customers who have at least one order
SELECT c.name, o.total
FROM customers c
INNER JOIN orders o ON o.customer_id = c.id;

-- LEFT: every customer, even those with zero orders (total = NULL)
SELECT c.name, o.total
FROM customers c
LEFT JOIN orders o ON o.customer_id = c.id;

-- Classic use: find customers with NO orders
SELECT c.name
FROM customers c
LEFT JOIN orders o ON o.customer_id = c.id
WHERE o.id IS NULL;   -- the LEFT JOIN's NULL side reveals the gap
```

## Ati-ati

- Nyaring tabel tengen ing `WHERE` (contoné `WHERE o.total > 100`) kanthi sepi ngowahi LEFT JOIN bali dadi INNER JOIN, merga `NULL > 100` iku salah. Selehna kahanan mangkono ing klausa `ON` yèn pengin njaga baris sing ora cocog.
- Tansah join ing kolom sing **di-index** (foreign key) utawa join mudhun dadi nested scan.

## Kenapa iki penting

Umumé query nyata mbentang pirang-pirang tabel, lan tipe join sing salah kanthi sepi ngeculaké utawa nggandhakaké baris — bug sing ngasilaké asil sing *katon mèmper*. Ngerti yèn LEFT JOIN + `IS NULL` nemokaké baris sing "ilang", lan yèn `WHERE` ing sisih null mbatalaké LEFT, nyegah sakabèhé klas kesalahan laporan.

Telu sing kokbutuhaké

Ati-ati

Kenapa iki penting

Pitakon sing gegandhengan