How does MySQL find the latest data row that meets the conditions?

Do business at ordinary times , It is often the latest data that needs to be checked .

As for the latest concept , For products , What I often say is Time sequence , The latest is Recent meaning .
 

 

With examples ：

This is a record sheet for recording the visits of personnel .
The data in the data sheet accurately records the color of the hat everyone wears when visiting 、 Time 、 Personnel code （ Everyone only ）.

 

Sample data ：

 

What needs to be done is ：

Take out the latest visit records that meet the conditions .

What would you do best ？ 

First realize a little , Take out A101 This person is encoded Latest visit records .

First show the wrong sql Example ： Take it for granted max() function .

SELECT MAX(id) AS id ,user_code,cap_color,create_time FROM vist_record WHERE user_code='A101' ; 

Query results （ The wrong result ）：

 

Obviously, at first glance, the data are similar .

But it's actually wrong , Is to filter according to the conditions , Then we choose the largest one that meets the conditions id value , Replaced separately id.
 

The correct data is ：

 

Is that max（id） I can't use it ？

Proper use （ Will meet the conditions of the maximum id Value as a condition ）：

SELECT
 id,user_code,cap_color,create_time
FROM vist_record
WHERE id IN (SELECT MAX(id) AS id FROM vist_record WHERE user_code='A101' )
 

Query results ：

 

But see the above method of using subqueries ,

Everyone must have secretly cursed their mother in their hearts , It's so troublesome to get the latest data ？

Do you have something simpler ？

Yes .

for instance , We have made sure that , id Since the increase ,id The biggest data （ Eligible data ） It's the latest .

Then we can use reverse order DESC To get the latest data ：

DESC That is to say In reverse order / Descending .

PS：

  Use reverse order to find ：

SELECT *
FROM vist_record
WHERE user_code='A101'
ORDER BY id DESC
LIMIT 1;

Query results ： 

  Or in reverse chronological order ：

SELECT *
FROM vist_record
WHERE user_code='A101'
ORDER BY create_time DESC
LIMIT 1;

Query results ： 

 

Is it so simple to implement ？ 

Then if what we need is not to specify A101 What we need is the latest data of everyone involved ？

That is, there are multiple groups . 

The latest qualified data of each category

The orange box is A101 、B202 、 C303 The latest record of each , We need to take it out .

 

 

 

The wrong sample ：
 

SELECT MAX(id) AS id ,user_code,cap_color,create_time FROM vist_record   GROUP BY user_code

Wrong filter results ：

 

Code correctly ：
 

SELECT  id ,user_code,cap_color,create_time FROM vist_record  WHERE id in
(
SELECT MAX(id) AS id  FROM vist_record  GROUP BY user_code 
)

 

Okay , Let's stop here first .