protected void clone(Q query) {
        this.union = query.union;
        this.unionAll = query.unionAll;
        this.firstUnionSubQuery = query.firstUnionSubQuery;
    }
Creates an union expression for the given subqueries

@param <RT>
@param alias alias for union
@param sq subqueries
@return the current object
querydsl-sql/src/main/java/com/querydsl/sql/ProjectableSQLQuery.java