@SuppressWarnings("unchecked")
    private <RT> Union<RT> innerUnion(List<SubQueryExpression<RT>> sq) {
        queryMixin.setProjection(sq.get(0).getMetadata().getProjection());
        if (!queryMixin.getMetadata().getJoins().isEmpty()) {
            throw new IllegalArgumentException("Don't mix union and from");
        }
        this.union = UnionUtils.union(sq, unionAll);
        this.firstUnionSubQuery = sq.get(0);
        return new UnionImpl(this);
    }
Add the given Expression as a query flag

@param position position
@param flag query flag
@return the current object
querydsl-sql/src/main/java/com/querydsl/sql/ProjectableSQLQuery.java