1. 题目

Table: Users

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| user_id       | int     |
| name          | varchar |
| mail          | varchar |
+---------------+---------+

user_id is the primary key for this table.
This table contains information of the users signed up in a website. Some e-mails are invalid.

Write an SQL query to find the users who have valid emails.

A valid e-mail has a prefix name and a domain where:

• The prefix name is a string that may contain letters (upper or lower case), digits, underscore '_', period '.' and/or dash '-'.
• The prefix name must start with a letter.
• The domain is '@leetcode.com'.

Return the result table in any order.

The query result format is in the following example.

Users
+---------+-----------+-------------------------+
| user_id | name      | mail                    |
+---------+-----------+-------------------------+
| 1       | Winston   | winston@leetcode.com    |
| 2       | Jonathan  | jonathanisgreat         |
| 3       | Annabelle | bella-@leetcode.com     |
| 4       | Sally     | sally.come@leetcode.com |
| 5       | Marwan    | quarz#2020@leetcode.com |
| 6       | David     | david69@gmail.com       |
| 7       | Shapiro   | .shapo@leetcode.com     |
+---------+-----------+-------------------------+Result table:
+---------+-----------+-------------------------+
| user_id | name      | mail                    |
+---------+-----------+-------------------------+
| 1       | Winston   | winston@leetcode.com    |
| 3       | Annabelle | bella-@leetcode.com     |
| 4       | Sally     | sally.come@leetcode.com |
+---------+-----------+-------------------------+
The mail of user 2 doesn't have a domain.
The mail of user 5 has # sign which is not allowed.
The mail of user 6 doesn't have leetcode domain.
The mail of user 7 starts with a period.

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/find-users-with-valid-e-mails
著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

2. 解题

语法规则：
https://www.runoob.com/mysql/mysql-regexp.html
https://www.cnblogs.com/zhaopanpan/p/10133224.html

^ 表示开头
+ 匹配一个或多个,不包括空
[] 表示集合里的任意一个
\\ 用于转义特殊字符
a{m,n} 匹配m到n个a，左侧不写为0，右侧不写为任意
$ 表示以什么为结尾

# Write your MySQL query statement below
select * from Users
where mail regexp '^[a-zA-Z]+[a-zA-Z0-9_\\./\\-]{0,}@leetcode\\.com$'
order by user_id

---

我的CSDN博客地址 https://michael.blog.csdn.net/

长按或扫码关注我的公众号（Michael阿明），一起加油、一起学习进步！