Detailed explanation of Order By multi-field sorting rules in MySQL-Mysql Tutorial-php.cn

Detailed explanation of Order By multi-field sorting rules in MySQL

小云云

Release： 2017-12-14 11:54:25

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Want to know what rules mysql order by sorting is based on? This article mainly introduces the Order By multi-field sorting rule code example in MySQL. The editor thinks it is quite good. I will share it with you here. Friends who need it can refer to it. I hope it can help everyone.

MySql order by single field

Create a test table as follows:

CREATE TABLE `a` ( 
 `code` varchar(255) DEFAULT NULL, 
 `name` varchar(255) DEFAULT NULL 
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

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INSERT into a values(&#39;中一&#39;, &#39;我&#39;); 
INSERT into a values(&#39;中二&#39;, &#39;你&#39;); 
INSERT into a values(&#39;高一&#39;, &#39;我是&#39;); 
INSERT into a values(&#39;高二&#39;, &#39;我们&#39;); 
INSERT into a values(&#39;高二&#39;, &#39;我的&#39;);

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The test statement is as follows:

-- hex()： 获取16进制字节码 
select name, hex(name) from a order by name desc

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The result is as follows:

##weE68891E4BBAC我E68891你E4BDA0

name	hex(name)
My	E68891E79A84
Me It’s	E68891E698AF

Obviously, sorting in MySQL is based on bytecode. When the first word is the same, compare the bytecode of the second word, and so on

MySql order by multiple fields

Only with comparison can we have ideas, and only with comparison can we make progress. Therefore, we first list the descending sorting results of a single field, and then look at the descending sorting results of two fields, and we can analyze the reason from it.

-- 按照name进行降序排序 
select * from a order by name desc; 
-- 按照code进行降序排序 
select * from a order by code desc;

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The left side is order by name desc, and the right side is the result of order by code desc

code name高二我的高一我是高二我们中一我中二你

##code高二高二高一中二中一The result is obvious: when a single field is sorted, the order in which other fields appear is naturally sorted.

name
我们
我
I am
你
我

Let’s take a look at multi-field sorting

-- 按照code, name进行降序排序 
select * from a order by code, name desc;

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code中一中二高一高二高二The results are as follows: First of all, thank you qq_27837327 and MjayTang. I tested it here. The original text said that the SQL sorting is invalid is wrong. In fact, order by code, name desc is equivalent to order by code asc, name desc

name
我
你
I am
我的
Our

After testing, it was found that the sorting effect of select * from an order by code and name desc is still invalid.

We are looking at the following statement

-- 按照code, name进行降序排序 
select * from a order by code desc, name desc; 
-- 该语句的效果等同于下面的语句, 其中1、2分别对应的是code、name 
select code, name from a order by 1 desc, 2 desc;

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code高二高二高一中二中一Comparing the descending order of single fields of code and name, we can find that when using order by code desc, name desc, MySQL will first perform the order by code. Sort in descending order. Based on the descending order of code, use name to sort in descending order.

name
我的
我们
I am
你
我

In addition, we can also use the contat function to splice multiple fields together and sort them. But make sure the field cannot be null. Let's take a look at the sql statement and results of concat.

select * from a order by concat(code,name) desc

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code高二高二高一中二中一Obviously, from this test example, the effect of order by concat(code, name) desc is equivalent to order by code desc, name desc

name
My
我们
我是
你
我

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