sql 语句

SQL（Structured Query Language，结构化查询语言）是一种用于管理关系数据库（RDBMS）的标准语言。以下是一些常见的 SQL 语句示例：

1. 创建表格：

CREATE TABLE customers (
    id INT PRIMARY KEY,
    name VARCHAR(50),
    email VARCHAR(50)
);

2. 插入数据：

INSERT INTO customers (id, name, email)
VALUES (1, 'John Smith', 'john@example.com');

3. 查询数据：

SELECT * FROM customers;

4. 更新数据：

UPDATE customers
SET email = 'john.smith@example.com'
WHERE name = 'John Smith';

5. 删除数据：

DELETE FROM customers
WHERE id = 1;

6. 连接表格：

SELECT orders.order_id, customers.name
FROM orders
INNER JOIN customers
ON orders.customer_id = customers.id;

7. 排序：

SELECT * FROM customers
ORDER BY name ASC;

8. 分组：

SELECT gender, COUNT(*) as count
FROM employees
GROUP BY gender;

9. 使用聚合函数：

SELECT AVG(salary) as avg_salary
FROM employees;

10. 使用子查询：

SELECT name
FROM customers
WHERE id IN (
    SELECT customer_id
    FROM orders
    WHERE order_date > '2020-01-01'
);
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