Convert a string to a number in oracle

To convert a string to a number in Oracle you use the Oracle / PLSQL: TO_NUMBER Function[^] (Try using your favourite search engine with the phrase "oracle convert string to number")

Your problem, as Maciej has pointed out, is that "1.234.56" is not a valid number in any culture that I am aware of. You would first need to remove that first period, and you can do that with REGEXP_REPLACE[^]

If it was meant to be "1,234.56" or "1.234,56" (German) or "1 234.56" (Swedish) then Oracle will still not recognise it as a number unless you do something about Globalisation - see Globalization Support[^]

Here is some sample code that demonstrates what might need to be done

with exampleData as (
  select '1.234.56' col1 from dual union all
  select '1234.56'  col1 from dual union all
  select '1 234.56'  col1 from dual union all
  select '1,234.56' col1 from dual 
)
select 
  case when length(col1) - length(replace(col1,'.',null)) > 1 
      then TO_NUMBER(regexp_replace(col1,'\.','',1,1))
  when length(col1) - length(replace(col1,',',null)) > 0 
      then TO_NUMBER(regexp_replace(col1,'\,','',1,1))
      -- Assuming not German!
  when length(col1) - length(replace(col1,' ',null)) > 0 
      then TO_NUMBER(regexp_replace(col1,' ','',1,1))
      -- Assuming not Swedish!
  else
      TO_NUMBER(col1) 
  end cnvrted
from exampleData;