最近一直在用spring-data-jpa这个东西，感觉方法上注解hql语句已经是很常用的方法了，
有一些关于hql的经验分享一下：
一.hql的join
hql的优势就是直接的关联关系嘛，但是通过hql语句进行join查询的时候，需要注意一些地方：
hql的join不能像类似sql的join一样，直接join两张表，需要先维护关联关系，在join：
@Entity
@Table(name = "mq_mark")
public class MarkModel implements Serializable {
/**
*
*/
private static final long serialVersionUID = -5169585172209348371L;
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
public Long id;
@ManyToOne(fetch = FetchType.EAGER)
@JoinColumn(name = "show_id", nullable = false)
@Fetch(FetchMode.JOIN)
public ShowImage showImage;
}
然后在进行hql join查询
@Query("select r from MarkModel t join t.showImage r where t.type= ?1 and t.datamId = ?2 ")
List<ShowImage> findMarkImages(Integer marktype, Long datamId, Pageable pageable);
会生成 left join 语句
二：hql语句的去重
使用sql去重一般是使用distinc ，但是这种方式是会影响效率的。
hql的去重推荐使用group by
@Query("select t.showImage from MarkModel t where t.type= ?1 and t.datamId = ?2 group by t.showImage.id")
List<ShowImage> findMarkImages(Integer marktype, Long datamId, Pageable pageable);
看一下其生成的语句：
select showimage1_.id as id1_25_, showimage1_.create_time as create_t5_25_, showimage1_.image_url as image_ur8_25_, showimage1_.position as positio14_25_
from xxx markmodel0_ inner join mq_showpic showimage1_ on markmodel0_.show_id=showimage1_.id
cross join mq_showpic showimage2_
where markmodel0_.show_id=showimage2_.id and markmodel0_.type=? and markmodel0_.datam_id=?
group by markmodel0_.show_id order by showimage2_.create_time desc limit ?
这里看到有一个cross join ，看见cross join就说明要生成笛卡尔积，这样会非常大的影响效率，
经过排查，发现只有order by的时候有用到了showimage2_,所以猜测是order by的问题，看下怎么生成的order by
Pageable pageable = new PageRequest(page, rows, Direction.DESC, "showImage.createTime");//注意这里的条件 List<ShowImage> result = this.markDao.findMarkImages(marktype, datamId, pageable);
这里使用了join的表进行排序，所以才出现了cross join
将这里改成用主表的createTime进行排序
Pageable pageable = new PageRequest(page, rows, Direction.DESC, "createTime");
生成的sql立马就变了：
select showimage1_.id as id1_25_, showimage1_.create_time as create_t5_25_, showimage1_.image_url as image_ur8_25_, showimage1_.position as positio14_25_
from mq_mark markmodel0_ inner join mq_showpic showimage1_ on markmodel0_.show_id=showimage1_.id
where markmodel0_.type=? and markmodel0_.datam_id=?
group by showimage1_.id order by markmodel0_.create_time desc limit ?
不在有cross join了
