# Riferimento Rapido SQL

*SELECT, JOIN, subquery, indici, transazioni*

> 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

### Operatori di Confronto

| Command | Description |
|---------|-------------|
| `= <> (!=)` | Uguale / diverso |
| `< > <= >=` | Operatori di confronto |
| `AND OR NOT` | Operatori logici |
| `IS NULL / IS NOT NULL` | Controlli null |

### Pattern Matching

```
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

### Tipi di Join

| Command | Description |
|---------|-------------|
| `INNER JOIN` | Righe corrispondenti in entrambe le tabelle |
| `LEFT JOIN` | Tutte le righe sinistre + corrispondenti destre |
| `RIGHT JOIN` | Tutte le righe destre + corrispondenti sinistre |
| `FULL OUTER JOIN` | Tutte le righe di entrambe le tabelle |
| `CROSS JOIN` | Prodotto cartesiano di entrambe le tabelle |

### Sintassi 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

### Sintassi

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

### Tipi di Dato Comuni

| Command | Description |
|---------|-------------|
| `INTEGER` | Numeri interi |
| `REAL` | Numeri in virgola mobile |
| `TEXT` | Dati stringa/testo |
| `BLOB` | Dati binari |
| `BOOLEAN` | TRUE / FALSE (memorizzato come 0/1) |
| `DATE / DATETIME` | Valori di data e timestamp |

### Vincoli

| Command | Description |
|---------|-------------|
| `PRIMARY KEY` | Identificatore univoco di riga |
| `NOT NULL` | Valore obbligatorio |
| `UNIQUE` | Nessun valore duplicato |
| `DEFAULT val` | Valore predefinito se omesso |
| `CHECK (expr)` | Regola di validazione personalizzata |
| `FOREIGN KEY` | Riferimento a un'altra tabella |

## Funzioni di Aggregazione

| Command | Description |
|---------|-------------|
| `COUNT(*)` | Numero di righe |
| `COUNT(col)` | Valori non-null nella colonna |
| `SUM(col)` | Somma della colonna numerica |
| `AVG(col)` | Media della colonna numerica |
| `MIN(col)` | Valore minimo |
| `MAX(col)` | Valore massimo |

### Esempio

```
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 filtra le righe prima del raggruppamento; HAVING filtra i gruppi dopo l'aggregazione*

## 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

### Nella Clausola WHERE

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

### Come Tabella Derivata

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

## Indici

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

### Quando Usare gli Indici

| Command | Description |
|---------|-------------|
| `Colonne in WHERE` | Velocizza il filtraggio |
| `Colonne in JOIN ON` | Velocizza le ricerche nei join |
| `Colonne in ORDER BY` | Velocizza l'ordinamento |
| `Colonne ad alta cardinalità` | Molti valori unici traggono maggior beneficio |