>It's valid SQL statement but it will only work when Field2 stores character representetaion of integer. See http://berezniker.com/content/pages/sql/microsoft-sql-server/isnumericex-udf-data-type-aware for more info.

Ok, that's right. I have adjusted the SQL to this:

UPDATE Client SET Field2=CAST(Field1 AS NUMERIC(20,0))

As Field2 is numeric(20,0).

Now, I have this message:

"Error converting data type varchat to numeric"