Function nth_value 

pub fn nth_value<'a, V, E>(
    expr: E,
    n: usize,
) -> WindowFnExpr<'a, V, E::SQLType, Null>
where
    V: SQLParam + 'a,
    E: Expr<'a, V>,

NTH_VALUE(expr, n) — value of expr from the nth row of the frame.

Always nullable (n may exceed frame size).