Function ifnull 

pub fn ifnull<'a, V, E, D>(
    expr: E,
    default: D,
) -> SQLExpr<'a, V, E::SQLType, Null, Scalar>
where
    V: SQLParam + 'a,
    E: Expr<'a, V>,
    D: Expr<'a, V>,
    E::SQLType: Compatible<D::SQLType>,

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.