Function avg 

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

AVG(expr) - calculates average of numeric values.

Requires the expression to be Numeric. Always returns Double (SQL standard behavior), nullable.

Type Safety

// ✅ OK: Numeric column
avg(products.price);

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