sub query

SubQueryExpression은 sub query를 표현합니다. 아래와 같이 subQuery를 선언합니다.

import static com.querydsl.sql.SQLExpressions.select;
import com.querydsl.core.types.SubQueryExpression;

QSurvey survey = QSurvey.survey;
SubQueryExpression<String> subQuery = select(survey.name)
  .from(survey);

그리고 sub query의 별명을 사용하기 위해 아래와 같이 선언합니다.

import static com.querydsl.core.types.dsl.Expressions.stringPath;
import com.querydsl.core.types.dsl.StringPath;

final StringPath subQueryAlias = stringPath("subQueryAlias"); 


QEmployee employee = QEmployee.employee;

return queryFactory
  .select(employee.id)
  .from(employee)
  .join(subQuery, subQueryAlias)
  .on(employee.name.eq(stringPath(subQueryAlias, "name")))
  .fetch();

multi table union sub query 

import static com.querydsl.sql.SQLExpressions.unionAll;

QEmployee employee = QEmployee.employee;
QSurvey survey = QSurvey.survey;
QPerson person = Qperson.person;

final String id = "id";
final String name = "name";

final StringPath unionQueryAlias = stringPath("unionQueryAlias");

SubQueryExpression<Tuple> employeeQuery = select(
    employee.id.as(id),
    employee.name.as(name))
    .from(employee);

SubQueryExpression<Tuple> surveyQuery = select(
    survey.employeeId.as(id),
    survey.name.as(name))
    .from(survey);

SubQueryExpression<Tuple> unionQuery = unionAll(employeeQuery, surveyQuery);

return queryFactory
  .select(
    numberPath(Long.class, unionQueryAlias, id),
    stringPath(unionQueryAlias, name))
  .from(unionQuery, unionQueryAlias)
  .join(person)
  .on(numberPath(Long.class, unionQueryAlias, id).eq(person.id))
  .fetch();