union联表

union操作用于合并多个查询结果，需要注意的是，多个查询结果的列数必须相同。在QtDao中，dao::_select()和dao::_join()两个类型都提供了unionSelect函数。

template<typename E>
unionSelect(Select<E>& select, bool unionAll = false);

template<typename... E>
unionSelect(Join<E...>& join, bool unionAll = false);

• union select

StudentTb::Field stuField;
TeacherTb::Field teaField;

auto query0 = dao::_select<StudentTb>()
    .column(stuField.name, stuField.score)
    .filter(stuField.score > 100)
    .build();

auto result = dao::_select<TeacherTb>()
    .column(teaField.name, teaField.score)
    .filter(teaField.age > 20)
    .unionSelect(query0)
    .build().list();

• union join

StudentTb::Field stuField;
TeacherTb::Field teaField;

auto query0 = dao::_join<StudentTb, TeacherTb>()
    .from<StudentTb>()
    .column(stuField.name, stuField.score)
    .innerJoin<TeacherTb>().on(teaField.id == stuField.teaId)
    .filter(stuField.score >= 90)
    .build();

auto result = dao::_select<StudentTb>()
    .column(stuField.name, stuField.score)
    .filter(stuField.score > 100)
    .unionSelect(query0)
    .build().list();