Function upper 

pub fn upper<'a, V, E>(
    expr: E,
) -> SQLExpr<'a, V, <V::DialectMarker as DialectTypes>::Text, E::Nullable, E::Aggregate>
where
    V: SQLParam + 'a,
    E: Expr<'a, V>,
    E::SQLType: Textual,

UPPER - converts string to uppercase.

Preserves the nullability of the input expression.

Type Safety

// ✅ OK: Text column
upper(users.name);

// ❌ Compile error: Int is not Textual
upper(users.id);