项目1：查找重复的电子邮箱

创建 email表，并插入如下三行数据

-- 创建表

CREATE TABLE email(
id INT NOT NULL auto_increment,
email VARCHAR(20) NOT NULL,
PRIMARY KEY(id));

-- 插入数据

INSERT into email(email) VALUES
("a123@.com"),
("b123@.com"),
("a123@.com"),
("d123@.com");

---查找重复的电子邮箱：

SELECT email
FROM email
GROUP BY email
HAVING COUNT(email)>1;