从MySQL中的表中提取最大值的最佳方法

Question

I have a mysql table where the details of check-in activity performed by all the users is captured. Below is the structure with sample data.

Table Name: check_in

+-----------+--------------------+---------------------+
|   id      |       user_id      |      time           |
+-----------+--------------------+---------------------+
|   1       |       10001        | 2016-04-02 12:04:02 |
|   2       |       10001        | 2016-04-02 11:04:02 |
|   3       |       10002        | 2016-10-27 23:56:17 |
|   4       |       10001        | 2016-04-02 10:04:02 |
|   5       |       10002        | 2016-10-27 22:56:17 |
|   6       |       10002        | 2016-10-27 21:56:17 |
+-----------+--------------------+---------------------+

On the dashboard, I have to display each user and at what time was their latest check-in activity performed (Sample dashboard view shown below).

User 1's last check-in was at 2016-04-02 12:04:02

User 2's last check-in was at 2016-10-27 23:56:17

What is the best and efficient way to write the query to pull this data? I have written below query, but it is taking 5-8 seconds to complete the execution. Note: This table has hundreds of thousands of rows.

select user_id, max(time) as last_check_in_at from check_in group by user_id

Answer 1

Your SQL query look optimized to me.

The reason for it being slow is probably that you do not have indexes on user_id and time columns.

Try adding the following indexes to your table:

ALTER TABLE `check_in` ADD INDEX `user_id` (`user_id`)
ALTER TABLE `check_in` ADD INDEX `time` (`time`)

and then execute your SQL query again to see if it makes a difference.

The indexes should allow the SQL engine to quickly group the relevant records by user_id and also quickly determine the maximum time .

Indexes will also help to quickly sort data by time (as suggested by Rajesh)

Answer 2

Simply use order by
Note:- please don't use time as a column name because may be it should be reserved keywords in any language

  select chk.user_id user_id, chk.time as last_check_in_at from   
    check_in chk group by chk.user_id ORDER BY chk.time 

Answer 3

You need to use order by in query.

Please try this query

select `user_id`,`time` as last_check_in_at from check_in group by user_id order by `time` DESC

It works for me.