Mysql常见的SQL语句格式及实用技巧

常用约束与修饰符
-- 主键约束（唯一标识）
PRIMARY KEY (column)
-- 唯一约束（值不能重复）
UNIQUE (column)
-- 非空约束
NOT NULL
-- 默认值
DEFAULT value
-- 自增（整数类型）
AUTO_INCREMENT

-- 创建数据库（存在则跳过）
CREATE DATABASE IF NOT EXISTS db_name;
-- 删除数据库（谨慎使用）
DROP DATABASE IF EXISTS db_name;
-- 查看所有数据库
SHOW DATABASES;
-- 切换数据库
USE db_name;

CREATE TABLE IF NOT EXISTS users (
  id INT PRIMARY KEY AUTO_INCREMENT,
  name VARCHAR(50) NOT NULL,
  age INT DEFAULT 18,
  email VARCHAR(100) UNIQUE,
  created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
);

-- 添加列
ALTER TABLE users ADD COLUMN phone VARCHAR(20);
-- 修改列类型
ALTER TABLE users MODIFY COLUMN age TINYINT;
-- 重命名列
ALTER TABLE users CHANGE COLUMN phone mobile VARCHAR(20);
-- 删除列
ALTER TABLE users DROP COLUMN mobile;

DROP TABLE IF EXISTS users;

-- 查看表结构
DESCRIBE users;
-- 查看建表语句
SHOW CREATE TABLE users;

-- 单条插入
INSERT INTO users (name, age, email)
VALUES ('Alice', 25, 'alice@example.com');
-- 批量插入
INSERT INTO users (name, age)
VALUES ('Bob', 30), ('Charlie', 22);
-- 插入查询结果
INSERT INTO users_backup (name, age)
SELECT name, age FROM users WHERE age > 25;

-- 基础查询
SELECT * FROM users;
-- 条件查询
SELECT name, age FROM users WHERE age > 25 AND name LIKE 'A%';
-- 去重查询
SELECT DISTINCT age FROM users;
-- 排序与分页
SELECT * FROM users ORDER BY age DESC LIMIT 10 OFFSET 20;
-- 聚合函数
SELECT COUNT(*), AVG(age), MAX(age) FROM users;
-- 分组查询
SELECT age, COUNT(*) FROM users GROUP BY age HAVING COUNT(*) > 1;

-- 单条件更新
UPDATE users SET age = age + 1 WHERE name = 'Alice';
-- 多字段更新
UPDATE users SET age = 30, email = 'bob@new.com' WHERE id = 2;
-- 批量更新（基于子查询）
UPDATE users u
JOIN user_scores s ON u.id = s.user_id
SET u.age = s.age + 5;

-- 按条件删除
DELETE FROM users WHERE age < 18;
-- 清空表（保留结构）
TRUNCATE TABLE users;

-- 内连接
SELECT u.name, o.order_id
FROM users u
JOIN orders o ON u.id = o.user_id;
-- 左连接
SELECT u.name, o.order_id
FROM users u
LEFT JOIN orders o ON u.id = o.user_id;

SELECT * FROM users
WHERE age > (SELECT AVG(age) FROM users);

SELECT name FROM users
UNION
SELECT product_name FROM products;

CREATE INDEX idx_age ON users (age)

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

-- 创建临时表并填充数据
CREATE TEMPORARY TABLE temp_orders AS
SELECT order_id, user_id, amount
FROM orders
WHERE create_time > '2023-01-01';
-- 使用临时表进行后续查询
SELECT user_id, SUM(amount)
FROM temp_orders
GROUP BY user_id;