Function variance 

pub fn variance<'a, V, E>(
    expr: E,
) -> SQLExpr<'a, V, <E::SQLType as StatisticalAggregatePolicy<V::DialectMarker>>::VarSamp, Null, Agg, E::Sources>
where
    V: SQLParam + 'a,
    E: Expr<'a, V>,
    E::SQLType: StatisticalAggregatePolicy<V::DialectMarker>,

Sample variance: VARIANCE on PostgreSQL, VAR_SAMP on MySQL.

Same typing as var_samp.

Examples

assert_eq!(
    variance(users.age).sql(),
    r#"CAST (VARIANCE ("users"."age") AS DOUBLE PRECISION)"#
);