Function ifnull 

pub fn ifnull<'a, V, E, D, N>(
    expr: E,
    default: D,
) -> SQLExpr<'a, V, E::SQLType, N, <E::Aggregate as AggOr<D::Aggregate>>::Output>
where
    V: SQLParam + 'a,
    E: Expr<'a, V>,
    D: Expr<'a, V>,
    N: Nullability,
    E::SQLType: Compatible<D::SQLType>,
    E::Nullable: NullAnd<D::Nullable, Output = N>,
    D::Nullable: Nullability,
    E::Aggregate: AggOr<D::Aggregate>,
    D::Aggregate: AggregateKind,

IFNULL - SQLite/MySQL equivalent of COALESCE with two arguments.

Requires compatible types between the expression and default. Returns the first argument if not NULL, otherwise returns the second.