QueryDSL

    <dependency>
    <groupId>com.querydsl</groupId>
    <artifactId>querydsl-apt</artifactId>
    <version>${querydsl.version}</version>  <!-- //可不寫，由 spring 自動判斷 --> 
    <scope>provided</scope>
    </dependency>

    <dependency>
    <groupId>com.querydsl</groupId>
    <artifactId>querydsl-jpa</artifactId>
    <version>${querydsl.version}</version>   <!-- //可不寫，由 spring 自動判斷 --> 
    </dependency>


    <!-- 會在 target/generated-sources/kotlin 生成 @Entity 的 Q類別 -->
    <project>
        <build>
            <plugins>
                ...
                <plugin>
                <groupId>com.mysema.maven</groupId>
                <artifactId>apt-maven-plugin</artifactId>
                <version>1.1.3</version>
                <executions>
                    <execution>
                    <goals>
                        <goal>process</goal>
                    </goals>
                    <configuration>
                        <outputDirectory>target/generated-sources/kotlin</outputDirectory>
                        <processor>com.querydsl.apt.jpa.JPAAnnotationProcessor</processor>
                    </configuration>
                    </execution>
                </executions>
                </plugin>
                ...
            </plugins>
        </build>
    </project>

    @Autowired
    lateinit var queryFactory: JPAQueryFactory

    val qadmin: QAdmin.admin

    bill = queryFactory.selectFrom(qadmin)
    .where(qadmin.account.eq("Bill"))
    .and(qadmin.password.ed("password"))
    .fetchOne();

    @Autowired
    private JPAQueryFactory queryFactory;

    QCat cat = QCat.cat;
    QCat mate = new QCat("mate");
    QCat kitten = new QCat("kitten");
    queryFactory.selectFrom(cat)
        .innerJoin(cat.mate, mate)
        .leftJoin(cat.kittens, kitten)
        .fetch();


    queryFactory.selectFrom(cat)
    .leftJoin(cat.kittens, kitten)
    .on(kitten.bodyWeight.gt(10.0))
    .fetch();

    QCustomer customer = QCustomer.customer;
    // rename customers named Bob to Bobby
    queryFactory.update(customer).where(customer.name.eq("Bob"))
        .set(customer.name, "Bobby")
        .execute();

    QCustomer customer = QCustomer.customer;
    // delete all customers
    queryFactory.delete(customer).execute();
    // delete all customers with a level less than 3
    queryFactory.delete(customer).where(customer.level.lt(3)).execute();

    QDepartment department = QDepartment.department;
    QDepartment d = new QDepartment("d");
    queryFactory.selectFrom(department)
        .where(department.size.eq(JPAExpressions.select(d.size.max()).from(d)))
        .fetch();



     //----------

    QEmployee employee = QEmployee.employee;
    QEmployee e = new QEmployee("e");
    queryFactory.selectFrom(employee)
        .where(employee.weeklyhours.gt(
            JPAExpressions.select(e.weeklyhours.avg())
                .from(employee.department.employees, e)
                .where(e.manager.eq(employee.manager))))
        .fetch();

    QCustomer customer = QCustomer.customer;
    queryFactory.selectFrom(customer)
        .orderBy(customer.lastName.asc(), customer.firstName.desc())
        .fetch();

    QCustomer customer = QCustomer.customer;
    queryFactory.select(customer.lastName).from(customer)
        .groupBy(customer.lastName)
        .fetch();