# Referencia Rápida de MySQL

*Bases de datos, tablas, CRUD, joins, índices, usuarios*

> Source: MySQL Documentation (dev.mysql.com) · MIT

## Conexión

### Línea de comandos

```
mysql -u root -p
mysql -u user -p -h 127.0.0.1 -P 3306
mysql -u user -p mydb < dump.sql
```

### Cadena de conexión

```
mysql://user:password@host:3306/dbname
mysql -u user -p -e "SELECT VERSION();"
```

### Comandos de estado

```
STATUS;
SHOW VARIABLES LIKE 'port';
SHOW PROCESSLIST;
```

## Bases de datos y tablas

### Operaciones de base de datos

```
CREATE DATABASE mydb CHARACTER SET utf8mb4;
SHOW DATABASES;
USE mydb;
DROP DATABASE mydb;
```

### Operaciones de tabla

```
CREATE TABLE users (
  id INT AUTO_INCREMENT PRIMARY KEY,
  name VARCHAR(100) NOT NULL,
  email VARCHAR(255) UNIQUE,
  created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
);
```

### Información de tabla

```
SHOW TABLES;
DESCRIBE users;
SHOW CREATE TABLE users;
```

### Modificar tabla

```
ALTER TABLE users ADD COLUMN age INT;
ALTER TABLE users MODIFY COLUMN name VARCHAR(200);
ALTER TABLE users DROP COLUMN age;
ALTER TABLE users RENAME TO customers;
```

## Tipos de datos

### Numéricos

| Command | Description |
|---------|-------------|
| `INT` | Entero de 4 bytes (-2B a 2B) |
| `BIGINT` | Entero de 8 bytes |
| `DECIMAL(p,s)` | Numérico exacto (p. ej., DECIMAL(10,2)) |
| `FLOAT / DOUBLE` | Punto flotante aproximado |
| `BOOLEAN` | Alias de TINYINT(1) |

### Cadenas

| Command | Description |
|---------|-------------|
| `VARCHAR(n)` | Cadena de longitud variable hasta n chars |
| `TEXT` | Texto de hasta 65 KB |
| `MEDIUMTEXT` | Texto de hasta 16 MB |
| `CHAR(n)` | Cadena de longitud fija, rellena |
| `ENUM('a','b')` | Un valor de un conjunto definido |

### Fecha y hora

| Command | Description |
|---------|-------------|
| `DATE` | YYYY-MM-DD |
| `DATETIME` | YYYY-MM-DD HH:MM:SS |
| `TIMESTAMP` | Fecha y hora almacenada en UTC |
| `TIME` | HH:MM:SS |
| `JSON` | Tipo de documento JSON nativo |

## CRUD

### Insertar

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

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

### Seleccionar

```
SELECT * FROM users WHERE id = 1;
SELECT name, email FROM users
ORDER BY name LIMIT 10 OFFSET 20;
```

### Actualizar

```
UPDATE users SET email = 'new@ex.com'
WHERE id = 1;
UPDATE users SET active = 0
WHERE last_login < '2025-01-01';
```

### Eliminar

```
DELETE FROM users WHERE id = 1;
TRUNCATE TABLE users;  -- fast, resets AUTO_INCREMENT
```

### Upsert

```
INSERT INTO users (id, name, email)
VALUES (1, 'Alice', 'a@ex.com')
ON DUPLICATE KEY UPDATE
name = VALUES(name), email = VALUES(email);
```

## Joins

### Tipos de join

| Command | Description |
|---------|-------------|
| `INNER JOIN` | Filas coincidentes en ambas tablas |
| `LEFT JOIN` | Todas las filas izquierdas + coincidentes derechas |
| `RIGHT JOIN` | Todas las filas derechas + coincidentes izquierdas |
| `CROSS JOIN` | Producto cartesiano |
| `SELF JOIN` | Tabla unida consigo misma |

### Ejemplos de join

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

SELECT e.name, m.name AS manager
FROM employees e
LEFT JOIN employees m ON e.manager_id = m.id;
```

## Índices

### Crear y eliminar

```
CREATE INDEX idx_name ON users(name);
CREATE UNIQUE INDEX idx_email ON users(email);
CREATE INDEX idx_composite
  ON orders(user_id, created_at);
DROP INDEX idx_name ON users;
```

### Índice de texto completo

```
ALTER TABLE posts ADD FULLTEXT(title, body);
SELECT * FROM posts
WHERE MATCH(title, body) AGAINST('mysql');
```

### Información de índices

```
SHOW INDEX FROM users;
EXPLAIN SELECT * FROM users WHERE name = 'Alice';
```

## Funciones

### Funciones de cadena

| Command | Description |
|---------|-------------|
| `CONCAT(a, b)` | Concatenar cadenas |
| `SUBSTRING(s, pos, len)` | Extraer subcadena |
| `UPPER(s) / LOWER(s)` | Cambiar mayúsculas/minúsculas |
| `TRIM(s)` | Eliminar espacios al inicio/final |
| `LENGTH(s)` | Longitud de cadena en bytes |
| `REPLACE(s, from, to)` | Reemplazar ocurrencias |

### Funciones de fecha

| Command | Description |
|---------|-------------|
| `NOW()` | Fecha y hora actual |
| `CURDATE()` | Fecha actual |
| `DATE_ADD(d, INTERVAL n DAY)` | Agregar intervalo a la fecha |
| `DATEDIFF(d1, d2)` | Diferencia en días |
| `DATE_FORMAT(d, fmt)` | Formatear fecha (p. ej., '%Y-%m-%d') |

### Funciones de agregación

```
SELECT COUNT(*), AVG(price), SUM(qty),
       MIN(price), MAX(price)
FROM products
GROUP BY category HAVING COUNT(*) > 5;
```

## Usuarios y permisos

### Gestión de usuarios

```
CREATE USER 'app'@'localhost'
IDENTIFIED BY 'secret';
ALTER USER 'app'@'localhost'
IDENTIFIED BY 'newsecret';
DROP USER 'app'@'localhost';
```

### Permisos

```
GRANT ALL ON mydb.* TO 'app'@'localhost';
GRANT SELECT, INSERT ON mydb.users
TO 'reader'@'%';
REVOKE INSERT ON mydb.users
FROM 'reader'@'%';
FLUSH PRIVILEGES;
```

### Ver permisos

```
SHOW GRANTS FOR 'app'@'localhost';
SELECT user, host FROM mysql.user;
```

## Backup y restauración

### mysqldump

```
mysqldump -u root -p mydb > backup.sql
mysqldump -u root -p --all-databases > all.sql
mysqldump -u root -p mydb users > users.sql
```

### Restaurar

```
mysql -u root -p mydb < backup.sql
mysql -u root -p -e "SOURCE /path/backup.sql"
```

### Registro binario

```
SHOW BINARY LOGS;
SHOW BINLOG EVENTS IN 'binlog.000001';
mysqlbinlog binlog.000001 | mysql -u root -p
```

## Patrones comunes

### Paginación

```
SELECT * FROM users
ORDER BY id LIMIT 20 OFFSET 40;  -- page 3
```

### Transacciones

```
START TRANSACTION;
UPDATE accounts SET balance = balance - 100
WHERE id = 1;
UPDATE accounts SET balance = balance + 100
WHERE id = 2;
COMMIT;  -- or ROLLBACK;
```

### Inserción condicional

```
INSERT IGNORE INTO users (email, name)
VALUES ('a@ex.com', 'Alice');
-- skips if email already exists (UNIQUE)
```

### Variables y sentencias preparadas

```
SET @name = 'Alice';
PREPARE stmt FROM
  'SELECT * FROM users WHERE name = ?';
EXECUTE stmt USING @name;
DEALLOCATE PREPARE stmt;
```