Update LOTES
Set EXIST = EXIST + (Select Sum(CANTIDAD) from LINEAS where
LOTES.CODIGO = LINEAS.CODIGO and LOTES.ARTICULO = LINEAS.ARTICULO)
where (Select Sum(CANTIDAD) from LINEAS where
LOTES.CODIGO = LINEAS.CODIGO and LOTES.ARTICULO = LINEAS.ARTICULO) <> 0