# SQL 快速参考

*SELECT、JOIN、子查询、索引与事务*

> 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

### 比较运算符

| Command | Description |
|---------|-------------|
| `= <> (!=)` | 等于 / 不等于 |
| `< > <= >=` | 比较运算符 |
| `AND OR NOT` | 逻辑运算符 |
| `IS NULL / IS NOT NULL` | Null 检查 |

### 模式匹配

```
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

### JOIN 类型

| Command | Description |
|---------|-------------|
| `INNER JOIN` | 两表中均匹配的行 |
| `LEFT JOIN` | 左表全部行 + 右表匹配行 |
| `RIGHT JOIN` | 右表全部行 + 左表匹配行 |
| `FULL OUTER JOIN` | 两表全部行 |
| `CROSS JOIN` | 两表的笛卡尔积 |

### JOIN 语法

```
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

### 插入

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

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

### 更新

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

### 删除

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

## CREATE TABLE

### 语法

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

### 常用数据类型

| Command | Description |
|---------|-------------|
| `INTEGER` | 整数 |
| `REAL` | 浮点数 |
| `TEXT` | 字符串 / 文本数据 |
| `BLOB` | 二进制数据 |
| `BOOLEAN` | TRUE / FALSE（存储为 0/1） |
| `DATE / DATETIME` | 日期与时间戳值 |

### 约束

| Command | Description |
|---------|-------------|
| `PRIMARY KEY` | 唯一行标识符 |
| `NOT NULL` | 值必填 |
| `UNIQUE` | 不允许重复值 |
| `DEFAULT val` | 省略时的默认值 |
| `CHECK (expr)` | 自定义验证规则 |
| `FOREIGN KEY` | 引用另一张表 |

## 聚合函数

| Command | Description |
|---------|-------------|
| `COUNT(*)` | 行数 |
| `COUNT(col)` | 列中的非 null 值数量 |
| `SUM(col)` | 数值列的总和 |
| `AVG(col)` | 数值列的平均值 |
| `MIN(col)` | 最小值 |
| `MAX(col)` | 最大值 |

### 示例

```
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 在分组前过滤行；HAVING 在聚合后过滤分组*

## 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
```

## 子查询

### 在 WHERE 子句中

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

### 作为派生表

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

## 索引

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

### 何时建索引

| Command | Description |
|---------|-------------|
| `WHERE 条件列` | 加速过滤 |
| `JOIN ON 列` | 加速 JOIN 查找 |
| `ORDER BY 列` | 加速排序 |
| `高基数列` | 唯一值越多，收益越大 |