Function ifnull 

pub fn ifnull<'a, V, E, D>(
    expr: E,
    default: D,
) -> SQLExpr<'a, V, E::SQLType, <E::Nullable as Nullability>::And<D::Nullable>, <E::Aggregate as AggregateKind>::Or<D::Aggregate>, Coalesce<Arg<<E as Expr<'a, V>>::Nullable, <E as ExprSources>::Sources>, Arg<<D as Expr<'a, V>>::Nullable, <D as ExprSources>::Sources>>>
where
    V: SQLParam + 'a,
    E: Expr<'a, V>,
    D: Expr<'a, V>,
    E::SQLType: Compatible<D::SQLType>,
    D::Nullable: Nullability,
    D::Aggregate: AggregateKind,

The first value, or default when it is NULL (IFNULL(expr, default)).

Same typing as coalesce. IFNULL exists on SQLite and MySQL but not on PostgreSQL. This function is not restricted by dialect, so on PostgreSQL it compiles but the database rejects it; use coalesce there.

Examples

let email = ifnull(users.email, "unknown");
assert_eq!(email.sql(), r#"IFNULL ("users"."email", ?)"#);