Function union_all 

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

Create a UNION ALL of multiple queries.

Returns None if the iterator is empty.

Example

let query = union_all([
    ("SELECT id FROM table1", vec![]),
    ("SELECT id FROM table2", vec![]),
    ("SELECT id FROM table3", vec![]),
]).expect("at least one query required");