Hi,
Try the following query...
select b.* from exlltala as a,Ghasht as b where
b.field3 like '%'+substring ('codposti',6,1)+'%'
Mistakes:
* Two times using 'from' keyword
* single quote missing in 'Char' value.
* Single quote missing using like operator.(Ex: like '%a%')
Cheers :)