hql和sql使用记录
hql：
String hql = "select new map(ct.curriculum.alias as curriculum_alias,ct.curriculum.name as curriculum_name) " + "from CurriculumTag ct where ct.id in (select min (ct.id) from CurriculumTag group by ct.curriculum.alias)";
该hql的语句的作用说起来比较饶，组织了好几次都感觉说不清楚，就这样吧。。。
修正：后来我试了一下上面的hql语句，查询结果不是我想要的。后来改为下面的hql语句才正确。其实下面这条hql就给嵌套查询的类加了一个别名
String hql = "select new map(ct.curriculum.alias as curriculum_alias,ct.curriculum.name as curriculum_name) " + "from CurriculumTag ct where ct.id in (select min (ct.id) from CurriculumTag ct group by ct.curriculum.alias)";
2014-04-16修改：
String hql = "select new map(ct.curriculum.alias as curriculum_alias,ct.curriculum.name as curriculum_name) " + "from CurriculumTag ct where ct.id in (select min(ct2.id) from CurriculumTag ct2 group by ct2.curriculum.alias)";
今天突然发现修改后的hql还是没得到我要的数据，又把嵌套查询的类的别名改为ct2，才正确工作了。原因是父查询的类的别名和子查询的类的别名不能一样，一个ct，一个ct2。。
注：distinct作用于一列和最用于多列的不同效果
表结构：
CREATE TABLE `tb_curriculum_tag` ( `id` bigint(20) NOT NULL auto_increment, `curriculum_alias` varchar(255) NOT NULL, `tag_alias` int(11) NOT NULL, PRIMARY KEY (`id`) ) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8;
表数据：
| id | curriculum_alias | tag_alias |
| 1 | 5F9FAGvQi6BJJBd | 665477 |
| 2 | 5F9FAGvQi6BJJBd | 294005 |
| 3 | 5F9FAGvQi6BJJBd | 828230 |
| 4 | 5F9FAGvQi6BJJBd | 356791 |
查询一：distinct作用于一列
mysql> select distinct curriculum_alias from tb_curriculum_tag -> ; +------------------+ | curriculum_alias | +------------------+ | 5F9FAGvQi6BJJBd | +------------------+ 1 row in set
查询二：distinct作用于多列
mysql> select distinct curriculum_alias,tag_alias from tb_curriculum_tag; +------------------+-----------+ | curriculum_alias | tag_alias | +------------------+-----------+ | 5F9FAGvQi6BJJBd | 665477 | | 5F9FAGvQi6BJJBd | 294005 | | 5F9FAGvQi6BJJBd | 828230 | | 5F9FAGvQi6BJJBd | 356791 | +------------------+-----------+ 4 rows in set
这就是不同的效果，查询二是把两个字段作为整体distinct，我的理解。。
查询三：count和distinct结合使用统计不重复列的个数
mysql> select count(distinct curriculum_alias) from tb_curriculum_tag; +----------------------------------+ | count(distinct curriculum_alias) | +----------------------------------+ | 1 | +----------------------------------+ 1 row in set
查询四：使用group by统计不重复列的个数
mysql> select count(*) from tb_curriculum_tag ct where ct.id in (select min(ct.id)from tb_curriculum_tag ct group by ct.curriculum_alias); +----------+ | count(*) | +----------+ | 1 | +----------+ 1 row in set
查询四使用group by也实现了查询三的效果。
查询五：使用group by的效果
mysql> select * from tb_curriculum_tag group by curriculum_alias -> ; +----+------------------+-----------+ | id | curriculum_alias | tag_alias | +----+------------------+-----------+ | 1 | 5F9FAGvQi6BJJBd | 665477 | +----+------------------+-----------+ 1 row in set
这里加上group by就查询出一条数据，和我原来想的不一样。group by主要结合一些统计函数来进行统计操作。
查询五：count和group by统计
mysql> select count(*) from tb_curriculum_tag ct where ct.id in (select ct.id from tb_curriculum_tag ct group by ct.curriculum_alias); +----------+ | count(*) | +----------+ | 1 | +----------+ 1 row in set
CurriculumTag类设计：
@Entity
@Table(name = "tb_curriculum_tag")
public class CurriculumTag implements Serializable {
private Long id;
private Curriculum curriculum;
private Tag tag;
////////////////setter and getter////////////////////
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
public Long getId() {
return id;
}
public void setId(Long id) {
this.id = id;
}
/**
* The name of the column referenced by this foreign
* key column.
*
* @return
*/
@ManyToOne(targetEntity = Curriculum.class)
@JoinColumn(name = "curriculum_alias", nullable = false, referencedColumnName = "alias")
public Curriculum getCurriculum() {
return curriculum;
}
public void setCurriculum(Curriculum curriculum) {
this.curriculum = curriculum;
}
@ManyToOne(targetEntity = Tag.class)
@JoinColumn(name = "tag_alias", nullable = false,referencedColumnName = "alias")
public Tag getTag() {
return tag;
}
public void setTag(Tag tag) {
this.tag = tag;
}
}