MySQL queries the latest qualified data rows

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 .

Why is it wrong , You can talk about it a little , Since someone in the comment area is interested （ Welcome brothers to express their views ）.

A brief account , max It's an aggregate function , Our error example doesn't match group by To use the , At this time, in fact mysql This guy can let us execute , Many databases report errors directly .

Then execution is execution , In fact, this time mysql It is equivalent to treating the entire table as a content block for a compressed search .

We added where Conditions   user_code='A101', So the whole content block does filter out other non user_code='A101' The data of .
That is to say, in the case of lax implementation ,mysql Guarantee max return （ Related column ） The maximum of , Other column fields are not guaranteed . 

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_timeFROM vist_recordWHERE 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_recordWHERE user_code='A101'ORDER BY id DESCLIMIT 1;

Query results ： 

  Or in reverse chronological order ：

SELECT *FROM vist_recordWHERE user_code='A101'ORDER BY create_time DESCLIMIT 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 )

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