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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