Function lag_with_default 

pub fn lag_with_default<'a, V, E, D>(
    expr: E,
    offset: usize,
    default: D,
) -> WindowFnExpr<'a, V, E::SQLType, <E::Nullable as NullOr<D::Nullable>>::Output>
where
    V: SQLParam + 'a,
    E: Expr<'a, V>,
    D: Expr<'a, V>,
    E::SQLType: Compatible<D::SQLType>,
    E::Nullable: NullOr<D::Nullable>,
    D::Nullable: Nullability,

LAG(expr, offset, default) — value of expr from N rows back with a default.

Nullability is the combination of the expression’s and default’s nullability.