山东做网站公司有哪些百度开户流程

前言

SQL（结构化查询语言）是数据库管理和操作的核心工具，无论是初学者还是经验丰富的数据库管理员，掌握常用的 SQL 语句对于高效管理和查询数据都至关重要。本文将系统性地介绍最常用的 SQL 语句，并为每个语句提供详细注释和实际案例，帮助大家在实际项目中得心应手地使用它们。

对于影刀RPA中级开发者，我们需要掌握基本SQL语句，增删改查，将数据插入到数据库存放，比传统的Excel文本存放要好的多，我们可以把数据集中管理，同时便于查询，不需要繁琐的合并数据！

数据定义语言（DDL）

数据库操作

• 创建数据库

  CREATE DATABASE my_database;
  CREATE DATABASE test_db CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;
  

• 删除数据库

  DROP DATABASE my_database;
  DROP DATABASE test_db;
  

• 选择数据库

  USE my_database;
  USE test_db;
  

表操作

• 创建表

  CREATE TABLE users (id INT PRIMARY KEY AUTO_INCREMENT,name VARCHAR(100) NOT NULL,email VARCHAR(100) UNIQUE NOT NULL
  );
  CREATE TABLE orders (order_id INT PRIMARY KEY AUTO_INCREMENT,user_id INT,amount DECIMAL(10, 2),FOREIGN KEY (user_id) REFERENCES users(id)
  );
  

• 删除表

  DROP TABLE users;
  DROP TABLE orders;
  

• 修改表

  ALTER TABLE users ADD COLUMN age INT;
  ALTER TABLE users MODIFY COLUMN email VARCHAR(150);
  ALTER TABLE users DROP COLUMN age;
  

数据操作语言（DML）

插入数据

• 插入单条数据

  INSERT INTO users (name, email, age) VALUES ('John Doe', 'john.doe@example.com', 30);
  

• 插入多条数据

  INSERT INTO users (name, email, age) VALUES
  ('Jane Smith', 'jane.smith@example.com', 25),
  ('Robert Brown', 'robert.brown@example.com', 40);
  INSERT INTO orders (user_id, amount) VALUES (1, 99.99);
  

删除数据

• 删除特定数据

  DELETE FROM users WHERE name = 'John Doe';
  DELETE FROM users WHERE age > 30;
  DELETE FROM orders WHERE amount < 50;
  

更新数据

• 更新特定数据

  UPDATE users SET age = 31 WHERE name = 'John Doe';
  UPDATE users SET age = age + 1;
  UPDATE orders SET amount = amount * 1.1 WHERE user_id = 1;
  

数据查询语言（DQL）

基础查询

• 查询所有记录

  SELECT * FROM users;
  SELECT * FROM orders;
  

• 查询特定列

  SELECT name, email FROM users;
  SELECT user_id, amount FROM orders;
  

排序查询

• 按列排序

  SELECT * FROM users ORDER BY age ASC;
  SELECT * FROM users ORDER BY age DESC;
  SELECT * FROM orders ORDER BY amount ASC;
  

聚合函数

• 计算总数

  SELECT COUNT(*) FROM users;
  SELECT COUNT(*) FROM orders;
  

• 计算平均值

  SELECT AVG(age) FROM users;
  SELECT AVG(amount) FROM orders;
  

• 计算总和

  SELECT SUM(amount) FROM orders;
  

• 计算最大值和最小值

  SELECT MAX(age) FROM users;
  SELECT MIN(amount) FROM orders;
  

分组查询

• 按列分组

  SELECT age, COUNT(*) FROM users GROUP BY age;
  SELECT user_id, SUM(amount) FROM orders GROUP BY user_id;
  

分页查询

• 分页查询

  SELECT * FROM users LIMIT 10 OFFSET 10;
  SELECT * FROM orders LIMIT 5 OFFSET 10;
  

连接查询

• 内连接查询

  SELECT users.name, orders.amount
  FROM users, orders
  WHERE users.id = orders.user_id;
  SELECT users.name, orders.amount
  FROM users
  INNER JOIN orders ON users.id = orders.user_id;
  

• 外连接查询

  SELECT users.name, orders.amount
  FROM users
  LEFT JOIN orders ON users.id = orders.user_id;
  SELECT users.name, orders.amount
  FROM users
  RIGHT JOIN orders ON users.id = orders.user_id;
  

子查询

• 子查询结果为单行单列

  SELECT *
  FROM users
  WHERE age = (SELECT MAX(age) FROM users);
  SELECT *
  FROM users
  WHERE id = (SELECT user_id FROM orders WHERE amount = (SELECT MAX(amount) FROM orders));
  

• 子查询结果为多行单列

  SELECT *
  FROM users
  WHERE age > (SELECT age FROM users WHERE age = 30);
  SELECT *
  FROM users
  WHERE id IN (SELECT user_id FROM orders);
  

• 子查询结果为多行多列

  SELECT *
  FROM users
  WHERE id IN (SELECT user_id FROM orders WHERE amount > 100);
  SELECT *
  FROM users
  WHERE id IN (SELECT user_id FROM orders WHERE amount > (SELECT AVG(amount) FROM orders));
  

数据控制语言（DCL）

管理用户

• 添加用户

  CREATE USER 'username'@'localhost' IDENTIFIED BY 'password';
  CREATE USER 'admin'@'localhost' IDENTIFIED BY 'securepass';
  

• 删除用户

  DROP USER 'username'@'localhost';
  DROP USER 'admin'@'localhost';
  

权限管理

• 查询权限

  SHOW GRANTS FOR CURRENT_USER;
  SHOW GRANTS FOR 'username'@'localhost';
  

• 授予权限

  GRANT ALL PRIVILEGES ON test_db.* TO 'username'@'localhost';
  GRANT SELECT, INSERT ON *.* TO 'admin'@'localhost';
  

• 撤销权限

  REVOKE ALL PRIVILEGES ON test_db.* FROM 'username'@'localhost';
  REVOKE SELECT, INSERT ON *.* FROM 'admin'@'localhost';
  

最后

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