Oracle String concatenation

The || operator does a String concatenation in Oracle.
You can use this in both SQL and pl/sql.
For example:

select kol1||kol2
from  tablename;

select kol1||' - '||kol2
from  tablename;

declare 
  l_var varchar2(2000);
begin
  l_var := 'abc'||'def';
end;

This is the short notation. You can also call the function 'concat'.
The concat function is limited to 2 parameters. With || you can concatenate a lot of strings together.

SELECT CONCAT(kol1,kol2)
from tablename;

declare 
  l_var varchar2(2000);
begin
  l_var := concat('abc','def');
end;