# Referensi Cepat SQL

*SELECT, JOIN, subquery, index, transaksi*

> Source: SQL Standard / SQLite Documentation (sqlite.org) · MIT

## SELECT

```
SELECT * FROM users;
SELECT name, email FROM users;
SELECT DISTINCT city FROM users;
SELECT name AS full_name FROM users;
```

## WHERE

### Operator Perbandingan

| Command | Description |
|---------|-------------|
| `= <> (!=)` | Sama / tidak sama |
| `< > <= >=` | Operator perbandingan |
| `AND OR NOT` | Operator logika |
| `IS NULL / IS NOT NULL` | Pengecekan null |

### Pencocokan Pola

```
SELECT * FROM users WHERE name LIKE 'A%';
-- % = any chars, _ = single char
SELECT * FROM users WHERE age IN (20, 25, 30);
SELECT * FROM users WHERE age BETWEEN 18 AND 30;
```

## JOIN

### Jenis Join

| Command | Description |
|---------|-------------|
| `INNER JOIN` | Baris yang cocok di kedua tabel |
| `LEFT JOIN` | Semua baris kiri + yang cocok dari kanan |
| `RIGHT JOIN` | Semua baris kanan + yang cocok dari kiri |
| `FULL OUTER JOIN` | Semua baris dari kedua tabel |
| `CROSS JOIN` | Hasil kartesian dari kedua tabel |

### Sintaks Join

```
SELECT u.name, o.total
FROM users u
INNER JOIN orders o ON u.id = o.user_id;

SELECT u.name, o.total
FROM users u
LEFT JOIN orders o ON u.id = o.user_id;
```

## INSERT / UPDATE / DELETE

### Insert

```
INSERT INTO users (name, email)
VALUES ('Alice', 'alice@example.com');

INSERT INTO users (name, email) VALUES
('Bob', 'bob@ex.com'),
('Carol', 'carol@ex.com');
```

### Update

```
UPDATE users SET email = 'new@ex.com'
WHERE id = 1;
```

### Delete

```
DELETE FROM users WHERE id = 1;
DELETE FROM users; -- delete all rows
```

## CREATE TABLE

### Sintaks

```
CREATE TABLE users (
  id    INTEGER PRIMARY KEY AUTOINCREMENT,
  name  TEXT NOT NULL,
  email TEXT UNIQUE,
  age   INTEGER DEFAULT 0,
  score REAL
);
```

### Tipe Data Umum

| Command | Description |
|---------|-------------|
| `INTEGER` | Bilangan bulat |
| `REAL` | Bilangan titik mengambang |
| `TEXT` | Data string / teks |
| `BLOB` | Data biner |
| `BOOLEAN` | TRUE / FALSE (disimpan sebagai 0/1) |
| `DATE / DATETIME` | Nilai tanggal dan timestamp |

### Constraint

| Command | Description |
|---------|-------------|
| `PRIMARY KEY` | Identifier baris yang unik |
| `NOT NULL` | Nilai wajib diisi |
| `UNIQUE` | Tidak ada nilai duplikat |
| `DEFAULT val` | Nilai default jika tidak diisi |
| `CHECK (expr)` | Aturan validasi kustom |
| `FOREIGN KEY` | Referensi ke tabel lain |

## Fungsi Agregat

| Command | Description |
|---------|-------------|
| `COUNT(*)` | Jumlah baris |
| `COUNT(col)` | Nilai non-null di kolom |
| `SUM(col)` | Jumlah kolom numerik |
| `AVG(col)` | Rata-rata kolom numerik |
| `MIN(col)` | Nilai minimum |
| `MAX(col)` | Nilai maksimum |

### Contoh

```
SELECT COUNT(*) AS total,
       AVG(age) AS avg_age,
       MAX(score) AS top_score
FROM users;
```

## GROUP BY / HAVING

```
SELECT city, COUNT(*) AS num_users
FROM users
GROUP BY city;

SELECT city, AVG(age) AS avg_age
FROM users
GROUP BY city
HAVING AVG(age) > 25;
```

*WHERE memfilter baris sebelum pengelompokan; HAVING memfilter grup setelah agregasi*

## ORDER BY / LIMIT

```
SELECT * FROM users ORDER BY name ASC;
SELECT * FROM users ORDER BY age DESC;
SELECT * FROM users
ORDER BY city, name
LIMIT 10 OFFSET 20;  -- skip 20, take 10
```

## Subquery

### Dalam Klausa WHERE

```
SELECT name FROM users
WHERE id IN (
  SELECT user_id FROM orders
  WHERE total > 100
);
```

### Sebagai Derived Table

```
SELECT city, avg_age FROM (
  SELECT city, AVG(age) AS avg_age
  FROM users GROUP BY city
) WHERE avg_age > 30;
```

## Index

```
CREATE INDEX idx_name ON users(name);
CREATE UNIQUE INDEX idx_email
  ON users(email);
DROP INDEX idx_name;
```

### Kapan Membuat Index

| Command | Description |
|---------|-------------|
| `Kolom di WHERE` | Mempercepat filtering |
| `Kolom di JOIN ON` | Mempercepat pencarian join |
| `Kolom di ORDER BY` | Mempercepat pengurutan |
| `Kolom high-cardinality` | Banyak nilai unik paling banyak untung |