# MySQL Quick Reference

*Databases, tables, CRUD, joins, indexes, users*

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

## Connecting

### Command Line

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

### Connection String

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

### Status Commands

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

## Databases & Tables

### Database Operations

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

### Table Operations

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

### Table Info

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

### Alter Table

```
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;
```

## Data Types

### Numeric

| Command | Description |
|---------|-------------|
| `INT` | 4-byte integer (-2B to 2B) |
| `BIGINT` | 8-byte integer |
| `DECIMAL(p,s)` | Exact numeric (e.g., DECIMAL(10,2)) |
| `FLOAT / DOUBLE` | Approximate floating-point |
| `BOOLEAN` | Alias for TINYINT(1) |

### String

| Command | Description |
|---------|-------------|
| `VARCHAR(n)` | Variable-length string up to n chars |
| `TEXT` | Up to 65 KB text |
| `MEDIUMTEXT` | Up to 16 MB text |
| `CHAR(n)` | Fixed-length string, padded |
| `ENUM('a','b')` | One value from a defined set |

### Date & Time

| Command | Description |
|---------|-------------|
| `DATE` | YYYY-MM-DD |
| `DATETIME` | YYYY-MM-DD HH:MM:SS |
| `TIMESTAMP` | UTC-stored datetime |
| `TIME` | HH:MM:SS |
| `JSON` | Native JSON document type |

## CRUD

### 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');
```

### Select

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

### Update

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

### Delete

```
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

### Join Types

| Command | Description |
|---------|-------------|
| `INNER JOIN` | Rows matching in both tables |
| `LEFT JOIN` | All left rows + matching right |
| `RIGHT JOIN` | All right rows + matching left |
| `CROSS JOIN` | Cartesian product |
| `SELF JOIN` | Table joined with itself |

### Join Examples

```
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;
```

## Indexes

### Create & Drop

```
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;
```

### Fulltext Index

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

### Index Info

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

## Functions

### String Functions

| Command | Description |
|---------|-------------|
| `CONCAT(a, b)` | Concatenate strings |
| `SUBSTRING(s, pos, len)` | Extract substring |
| `UPPER(s) / LOWER(s)` | Change case |
| `TRIM(s)` | Remove leading/trailing spaces |
| `LENGTH(s)` | String length in bytes |
| `REPLACE(s, from, to)` | Replace occurrences |

### Date Functions

| Command | Description |
|---------|-------------|
| `NOW()` | Current datetime |
| `CURDATE()` | Current date |
| `DATE_ADD(d, INTERVAL n DAY)` | Add interval to date |
| `DATEDIFF(d1, d2)` | Difference in days |
| `DATE_FORMAT(d, fmt)` | Format date (e.g., '%Y-%m-%d') |

### Aggregate Functions

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

## Users & Permissions

### User Management

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

### Grants

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

### View Permissions

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

## Backup & Restore

### mysqldump

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

### Restore

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

### Binary Log

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

## Common Patterns

### Pagination

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

### Transactions

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

### Conditional Insert

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

### Variables & Prepared Statements

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