Database daily question --- day 22: last login

One 、 Problem description

surface

: 

Logins

+----------------+----------+
|  Name  |  type  |
+----------------+----------+
| user_id | int |
| time_stamp | datetime |
+----------------+----------+
(user_id, time_stamp)  It's the primary key of this table .
The information contained in each line is user_id  The login time of this user .


Write a  

SQL

  Inquire about , The query can be obtained in  

2020

  Current year of all users logged in in   The last time  

The login time

. Result set   No   contain  

2020

  Users who haven't logged in in years .

The returned result set can be  

In any order

  array .

Query results

The format is as follows ：

Topic link

：

Last login

Two 、 Subject requirements

Examples

Input ：
Logins  surface :
+---------+---------------------+
| 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 |
+---------+---------------------+
Output ：
+---------+---------------------+
| 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 |
+---------+---------------------+
explain ：
6 User number is logged in 3 Time , But in 2020 Only once a year , Therefore, the result set should include this login .
8 User number is 2020 Logged in in 2 Time , Once in 2 month , Once in 12 month , therefore , The result set should contain 12 This login in June .
2 User number is logged in 2 Time , But in 2020 Only once a year , Therefore, the result set should include this login .
14 User number is 2020 I didn't log in in , Therefore, the result set should not contain .


Investigate

1. Aggregate functions
2. It is recommended to use time 10~25min


3、 ... and 、 Problem analysis

The title requires us to find

2020

Last logged in user in , Among them, users may log in several times in a year , We just need to find out

2020

Maximum date of the year .

For this question , Grouping aggregate functions can still solve the problem , We group by user number , Each number data after grouping contains the date of login , That's when we use

max

Function directly solves the maximum date .

Four 、 coded

select user_id, max(time_stamp) as 'last_stamp'
from Logins
where year(time_stamp)=2020
group by user_id


5、 ... and 、 test result