Function sum 

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

SUM(expr) - sums numeric values.

Requires the expression to be Numeric (Int, BigInt, Float, Double). Preserves the input expression’s SQL type. Returns a nullable expression (empty set returns NULL).

Type Safety

// ✅ OK: Numeric column
sum(orders.amount);
// Returns the same SQL type as orders.amount

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