Help with Mysql Query - Subindex_string + group_concat

Hi guys, 

New to CI and having some trouble with a few more advanced SQL queries . 

I am trying to use substring_index together with group_concat in order to make an array where I have 2 results of each unique group . 

I have tried the following :

Code:

$Array = $MyModel
   ->where('Column3 !=', 0)
   ->where('SUBSTRING_INDEX(GROUP_CONCAT(GROUP)),',1)
   ->orderBy('date', 'DESC')
   ->groupBy('GROUP')
   ->limit(2)
   ->find();

Could someone kindly help me fixing my query syntax ? 

Thanks in advance !