PSQL 第二天
💡
原文英文，约300词，阅读约需1分钟。
📝
内容提要
本文介绍了基本的SQL命令及示例，包括创建数据库、选择数据库、创建表、插入数据、查询、更新和删除记录，帮助用户有效管理数据库。
🎯
关键要点
- 创建数据库命令：CREATE DATABASE mydatabase;
- 选择数据库命令：USE mydatabase;
- 创建表命令：CREATE TABLE employees (id INT PRIMARY KEY, name VARCHAR(50), age INT, department VARCHAR(50));
- 插入数据命令：INSERT INTO employees (id, name, age, department) VALUES (1, 'Alice', 30, 'HR');
- 查询数据命令：SELECT * FROM employees; SELECT name, age FROM employees WHERE department = 'HR';
- 更新记录命令：UPDATE employees SET age = 31 WHERE id = 1;
- 删除记录命令：DELETE FROM employees WHERE id = 1;
- 删除表命令：DROP TABLE employees;
- 修改表结构命令：ALTER TABLE employees ADD salary DECIMAL(10,2);
- 修改列命令：ALTER TABLE employees MODIFY COLUMN age SMALLINT;
- 删除列命令：ALTER TABLE employees DROP COLUMN salary;
- 连接表命令：SELECT employees.name, departments.department_name FROM employees INNER JOIN departments ON employees.department = departments.id;
- 分组与过滤命令：SELECT department, COUNT() as total_employees FROM employees GROUP BY department HAVING COUNT() > 5;
- 排序结果命令：SELECT * FROM employees ORDER BY age DESC;
- 限制结果数量命令：SELECT * FROM employees LIMIT 5;
- 合并查询结果命令：SELECT name FROM employees UNION SELECT name FROM managers;
- 检查数据存在性命令：SELECT name FROM employees WHERE EXISTS (SELECT 1 FROM departments WHERE department_name = 'IT');
➡️
