compile method
Implementation
@override
String compile(Set<String> trampoline,
{bool includeTableName = false,
String? preamble,
bool withFields = true}) {
var selector = all == true ? 'UNION ALL' : 'UNION';
var t1 = Set<String>.from(trampoline);
var t2 = Set<String>.from(trampoline);
return '(${left.compile(t1, includeTableName: includeTableName)}) $selector (${right.compile(t2, includeTableName: includeTableName)})';
}