MySQL的substring_index函数简析
 
假定有一张表，其中有个字段是 email 保存了电子邮箱，然后，我想统计每种邮箱的占比情况。
我们知道电子信箱的格式是： account@domain ，要实现上面的统计，必须取得 email
字符串中 @ 后面的域名部分。
MySQL提供了一个名为 substring_index 的函数，能够满足我们的需求：
MySQL 5.5 Reference Manual 写道  www.2cto.com  
SUBSTRING_INDEX(str,delim,count)
 
Returns the substring from string str before count occurrences of the delimiter delim.
If count is positive, everything to the left of the final delimiter (counting from the left)
is returned. If count is negative, everything to the right of the final delimiter (counting
from the right) is returned. SUBSTRING_INDEX() performs a case-sensitive match when
searching for delim.
 
> SELECT SUBSTRING_INDEX('www.mysql.com', '.', 2);
 
        -> 'www.mysql'
 
mysql> SELECT SUBSTRING_INDEX('www.mysql.com', '.', -2);
 
        -> 'mysql.com'
 
This function is multi-byte safe. 
   www.2cto.com  
查询语句如下：
Sql代码  
select  
    substring_index(email,'@',-1) as domain,  
    count(distinct email) as count,  
    count(distinct email)  
        / (select count(distinct email) from qzt_resume_basic where email like '%@%')  
        * 100 as percent  
from qzt_resume_basic  
where email like '%@%'   
group by 1   
order by 2 desc   
limit 10;