Function sum 

pub fn sum<'a, V, E>(
    expr: E,
) -> SQLExpr<'a, V, <E::SQLType as AggregatePolicy<V::DialectMarker>>::Sum, Null, Agg>
where
    V: SQLParam + 'a,
    E: Expr<'a, V>,
    E::SQLType: AggregatePolicy<V::DialectMarker>,

SUM(expr) - sums numeric values.

Requires the expression to be Numeric (Int, BigInt, Float, Double). Result type is dialect-aware. Returns a nullable expression (empty set returns NULL).

Type Safety

// ✅ OK: Numeric column
sum(orders.amount);
// SQLite: same width for integer sums
// PostgreSQL: Int/SmallInt promote to BigInt

// ❌ Compile error: Text is not Numeric
sum(users.name);