find-users-with-valid-e-mails.sql

/*

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.

*/


# V0

# V1
# https://zhuanlan.zhihu.com/p/164674992
select * from Users 
where regexp_like(mail, '^[A-Za-z]+[A-Za-z0-9\_\.\-]*@leetcode.com')

# V2
# Time:  O(n)
# Space: O(n)
SELECT * 
FROM   users AS u 
WHERE  u.mail REGEXP '^[a-zA-Z][a-zA-Z0-9._-]*@leetcode.com$';