用SQlite设计邮件管理系统

需要创建数据表及相关代码，还有主键外键的约束，对表格的增删改查代码，创建视图和触发器。

下面是使用 SQLite 设计邮件管理系统的代码，包括创建数据表、主键和外键约束、增删改查代码、和创建视图和触发器。

1.创建数据表

CREATE TABLE users (
  id INTEGER PRIMARY KEY,
  name TEXT NOT NULL,
  email TEXT NOT NULL
);

CREATE TABLE emails (
  id INTEGER PRIMARY KEY,
  sender_id INTEGER NOT NULL,
  receiver_id INTEGER NOT NULL,
  subject TEXT NOT NULL,
  content TEXT NOT NULL,
  time TIMESTAMP NOT NULL,
  FOREIGN KEY (sender_id) REFERENCES users(id),
  FOREIGN KEY (receiver_id) REFERENCES users(id)
);

上面的代码创建了两个表：users 表用于存储用户信息，包括用户的 ID、名称和电子邮件地址；emails 表用于存储邮件信息，包括邮件的 ID、发件人、收件人、主题、正文和时间戳。同时，对于 emails 表中的 sender_id 和 receiver_id 列，还定义了对 users 表的外键约束。
2. 数据表的增删改查代码
添加用户：

INSERT INTO users (id, name, email) VALUES (1, '张三', 'zhangsan@example.com');

添加邮件：

INSERT INTO emails (id, sender_id, receiver_id, subject, content, time)
VALUES (1, 1, 2, '关于会议时间的通知', '各位同事：会议时间改为下周三上午10点，请准时参加。', '2023-06-20 10:30:00');

查询用户：

SELECT * FROM users;

查询邮件：

SELECT emails.id, sender.name AS sender, receiver.name AS receiver, subject, content, time
FROM emails
JOIN users AS sender ON emails.sender_id = sender.id
JOIN users AS receiver ON emails.receiver_id = receiver.id;

更新用户：

```sql
UPDATE users SET email = 'new-email@example.com' WHERE id = 1;

删除用户：

DELETE FROM users WHERE id = 1;

3. 创建视图
   创建一个名为 email_details 的视图，用于查看所有邮件的完整内容，包括发件人姓名、收件人姓名等信息。

CREATE VIEW email_details AS
SELECT emails.id, sender.name AS sender, receiver.name AS receiver, subject, content, time
FROM emails
JOIN users AS sender ON emails.sender_id = sender.id
JOIN users AS receiver ON emails.receiver_id = receiver.id;

可以使用下面的 SQL 语句查询这个视图：

SELECT * FROM email_details;

4.创建触发器
创建一个名为 update_email_count 的触发器，用于在更新邮件信息时同时更新发件人和收件人的邮件总数。当 emails 表中的记录被更新时，触发器会自动更新对应的用户的邮件数量。

CREATE TRIGGER update_email_count AFTER INSERT ON emails
BEGIN
  UPDATE users SET email_count = (SELECT COUNT(*) FROM emails WHERE sender_id = users.id) WHERE id = NEW.sender_id;
  UPDATE users SET email_count = (SELECT COUNT(*) FROM emails WHERE receiver_id = users.id) WHERE id = NEW.receiver_id;
END;

如果需要删除邮件，还需要添加一个名为 delete_email_count 的触发器，起到类似的作用。

CREATE TRIGGER delete_email_count AFTER DELETE ON emails
BEGIN
  UPDATE users SET email_count = (SELECT COUNT(*) FROM emails WHERE sender_id = users.id) WHERE id = OLD.sender_id;
  UPDATE users SET email_count = (SELECT COUNT(*) FROM emails WHERE receiver_id = users.id) WHERE id = OLD.receiver_id;
END;

以上是 SQLite 设计邮件管理系统的代码，希望能对您有所帮助，如有帮助希望采纳