将MySQL查询结果按值排序的简要教程_MySQL
MySQL查询结果如何排序呢?这是很多人都提过的问题,下面就教您如何对MySQL查询结果按某值排序,如果您感兴趣的话,不妨一看。
之前有一个功能修改,要求MySQL查询结果中:
id name * * * 1 lucy ... 3 lucy ... 2 lily ... 4 lucy ...
名字为lucy的优先排在前面,百思不得其解,可能有人会说简单 union嘛 或者弄个临时表什么的,其实我也想过,但是本身SQL逻辑就很多了(上面只是简例),再union的话或者临时表可能绕很大的弯路,后来看到一篇文章尝试着加入order by find_in_set(name,'lucy') ,结果 得到的结果为lucy全部在下面,随即我改为order by find_in_set(name,'lucy') desc 实现结果为
id name * * * 1 lucy ... 3 lucy ... 4 lucy ... 2 lily ...
基本实现,可是又有点不确定的心情,查mysql文档发现find_in_set语法
FIND_IN_SET(str,strlist)
假如字符串str 在由N 子链组成的字符串列数据表strlist 中, 则返回值的范围在 1 到 N 之间 。一个字符串列数据表就是一个由一些被『,』符号分开的自链组成的字符串。如果第一个参数是一个常数字符串,而第二个是type SET列,则 FIND_IN_SET() 函数被优化,使用比特计算。如果str不在strlist 或strlist 为空字符串,则返回值为 0 。如任意一个参数为NULL,则返回值为 NULL。 这个函数在第一个参数包含一个逗号(『,』)时将无法正常运行
mysql> SELECT FIND_IN_SET('b','a,b,c,d'); -> 2
看了这个我估计结果为什么要加desc 了 find_in_set返回的值是,当存在lucy的时候 返回他的位置,没有的时候为0,空的时候null,所以排序为1,1,1,0,如果加在列上就为
id name FIND_IN_SET * *
1 lucy 1 ... 3 lucy 1 ... 2 lily 0 ... 4 lucy 1...
表结构如下:
mysql> select * from test;
+----+-------+ | id | name | +----+-------+ | 1 | test1 | | 2 | test2 | | 3 | test3 | | 4 | test4 | | 5 | test5 | +----+-------+
执行以下SQL:
mysql> select * from test where id in(3,1,5);
+----+-------+ | id | name | +----+-------+ | 1 | test1 | | 3 | test3 | | 5 | test5 | +----+-------+ 3 rows in set (0.00 sec)
这个select在mysql中得结果会自动按照id升序排列,
如果我们
select * from table where id IN (3,6,9,1,2,5,8,7);
这样的情况取出来后,其实,id还是按1,2,3,4,5,6,7,8,9,排序的,但如果我们真要按IN里面的顺序排序怎么办?SQL能不能完成?是否需要取回来后再foreach一下?
其实可以这样
select * from table where id IN (3,6,9,1,2,5,8,7) order by field(id,3,6,9,1,2,5,8,7);
出来的顺序就是指定的顺序了
关于这种排序的效率,
有文章指出:
FIELD(str,str1,str2,str3,…) Returns the index (position) of str in the str1, str2, str3, … list. Returns 0 if str is not found.
排序过程:把选出的记录的 id 在 FIELD 列表中进行查找,并返回位置,以位置作为排序依据。
这样的用法,会导致 Using filesort,是效率很低的排序方式。除非数据变化频率很低,或者有长时间的缓存,否则不建议用这样的方式排序。
作者建议在程序代码中自行排序。
但是也有人说这种排序不会出现什么性能瓶颈
具体还是自己测试一下吧。
以上就是将MySQL查询结果按值排序的简要教程_MySQL的内容,更多相关内容请关注PHP中文网(www.php.cn)!

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