Leetcode1517.Find Users With Valid E-Mails （简单）

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   | [email protected]    |
| 2       | Jonathan  | jonathanisgreat         |
| 3       | Annabelle | [email protected]     |
| 4       | Sally     | [email protected] |
| 5       | Marwan    | quarz#[email protected] |
| 6       | David     | [email protected]       |
| 7       | Shapiro   | [email protected]     |
+---------+-----------+-------------------------+

Result table:

+---------+-----------+-------------------------+
| user_id | name      | mail                    |
+---------+-----------+-------------------------+
| 1       | Winston   | [email protected]    |
| 3       | Annabelle | [email protected]     |
| 4       | Sally     | [email protected] |
+---------+-----------+-------------------------+

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.

解答
这个就是在考察正则表达式吧
要求
前缀名称是一个字符串，可能包含字母（大写或小写），数字，下划线“ _”，句点. 和/或破折号-。
前缀名称必须以字母开头。
域是“ @ leetcode.com”。

^ 表示开头
$ 表示以什么为结尾
+ 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

.与_需要转义

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