mysql中多个前缀号码的提取方法

比如电话号码为：1234567（号码可能为8位或4位或7位）
前缀有：852，3，3852，852133
假如用户输入：31234567 ，在mysql中如何判断前缀是3？
或者用户输入：8521234567 。

 mysql> create table areacode(  
  ->  acode   varchar(10) primary key  
  -> )  
  ->  
  -> ;  
 Query OK, 0 rows affected (0.09 sec)  
   
 mysql> insert into areacode values (852),(3),(3852),(852133);  
 Query OK, 4 rows affected (0.05 sec)  
 Records: 4  Duplicates: 0  Warnings: 0  
   
 mysql> select * from areacode;  
 +--------+  
 | acode  |  
 +--------+  
 | 3   |  
 | 3852   |  
 | 852 |  
 | 852133 |  
 +--------+  
 4 rows in set (0.00 sec)  
   
 mysql> select substr('31234567',max(length(acode))+1)  
  -> from areacode  
  -> where instr('31234567',acode)=1 ;  
 +-----------------------------------------+  
 | substr('31234567',max(length(acode))+1) |  
 +-----------------------------------------+  
 | 1234567         |  
 +-----------------------------------------+  
 1 row in set (0.00 sec)  
   
 mysql> select substr('38521234567',max(length(acode))+1)  
  -> from areacode  
  -> where instr('38521234567',acode)=1 ;  
 +--------------------------------------------+  
 | substr('38521234567',max(length(acode))+1) |  
 +--------------------------------------------+  
 | 1234567         |  
 +--------------------------------------------+  
 1 row in set (0.05 sec)
   
 DELIMITER $$  
   
 DROP PROCEDURE IF EXISTS `CallCentre`.`test`$$  
   
 CREATE DEFINER=`edmond`@`localhost` PROCEDURE `test`(DialedNumbers char(20))  
 begin  
 Declare  RealDial char(20);  
 DECLARE RetVar char(20);  
   
 select substr(DialedNumbers,max(length(acode))+1) into RealDial  
   from areacode  
   where instr(DialedNumbers,acode)=1 ;  
 if not isnull(RealDial) then   
  set RetVar=RealDial; 
 else  
   set RetVar=DialedNumbers;  
 end if; 
   
  select RetVar;  
 end$$  
   
 DELIMITER ;  

 call test('85210000')