Function union 

pub fn union<I, S>(queries: I) -> Option<SetOperation>
where
    I: IntoIterator<Item = (S, Vec<Value>)>,
    S: Into<String>,

Create a UNION of multiple queries.

Returns None if the iterator is empty.

Example

let query = union([
    ("SELECT * FROM users WHERE role = 'admin'", vec![]),
    ("SELECT * FROM users WHERE role = 'manager'", vec![]),
]).expect("at least one query required");