@Override
    public <R,C> R accept(Visitor<R,C> v, @Nullable C context) {
        if (union != null) {
            return union.accept(v, context);
        } else {
            return super.accept(v, context);
        }
    }
{@code ProjectableSQLQuery} is the base type for SQL query implementations

@param <T> result type
@param <Q> concrete subtype
querydsl-sql/src/main/java/com/querydsl/sql/ProjectableSQLQuery.java