the-latest-login-in-2020.sql

/*

Table: Logins

+----------------+----------+
| Column Name    | Type     |
+----------------+----------+
| user_id        | int      |
| time_stamp     | datetime |
+----------------+----------+
(user_id, time_stamp) is the primary key for this table.
Each row contains information about the login time for the user with ID user_id.
Write an SQL query to report the latest login for all users in the year 2020. Do not include the users who did not login in 2020.

Return the result table in any order.

The query result format is in the following example:

Logins table:
+---------+---------------------+
| user_id | time_stamp          |
+---------+---------------------+
| 6       | 2020-06-30 15:06:07 |
| 6       | 2021-04-21 14:06:06 |
| 6       | 2019-03-07 00:18:15 |
| 8       | 2020-02-01 05:10:53 |
| 8       | 2020-12-30 00:46:50 |
| 2       | 2020-01-16 02:49:50 |
| 2       | 2019-08-25 07:59:08 |
| 14      | 2019-07-14 09:00:00 |
| 14      | 2021-01-06 11:59:59 |
+---------+---------------------+

Result table:
+---------+---------------------+
| user_id | last_stamp          |
+---------+---------------------+
| 6       | 2020-06-30 15:06:07 |
| 8       | 2020-12-30 00:46:50 |
| 2       | 2020-01-16 02:49:50 |
+---------+---------------------+

User 6 logged into their account 3 times but only once in 2020, so we include this login in the result table.
User 8 logged into their account 2 times in 2020, once in February and once in December. We include only the latest one (December) in the result table.
User 2 logged into their account 2 times but only once in 2020, so we include this login in the result table.
User 14 did not login in 2020, so we do not include them in the result table.
Explanation
Use max(), where and group by.

*/

# V0
select 
user_id, 
max(time_stamp) as last_stamp 
from Logins 
WHERE 
time_stamp >= '2020-01-01'
and time_stamp < '2021-01-01' 
group by user_id

# V1
# https://goodtecher.com/leetcode-1890-the-latest-login-in-2020/
# Write your MySQL query statement below
select 
user_id, 
max(time_stamp) as last_stamp 
from Logins 
WHERE 
time_stamp >= '2020-01-01'
and time_stamp < '2021-01-01' 
group by user_id

# V2
#Time:  O(n)
# Space: O(n)
SELECT user_id,
       MAX(time_stamp) AS last_stamp
FROM Logins
WHERE YEAR(time_stamp) = 2020
GROUP BY user_id
ORDER BY NULL;