hibernate之SQL查询

Posted 2020-09-09 code.world

一、SQL查询简介

使用SQL查询可以利用某些数据库的特性，或者将原有的JDBC应用迁移到hibernate应用上，也可能需要使用原生的SQL查询。查询步骤如下：

1、获取hibernate session对象

2、编写SQL语句

3、以SQL语句作为参数，调用Session的createSQLQuery()方法创建查询对象

4、调用SQLQuery对象的addScalar()huoaddEntity()方法将选出的结果与标量值或实体进行关联，分别用于进行标量查询或实体查询

5、如果SQL语句有参数，则调用Query的setXxx()方法为参数赋值

6、调用Query的list()方法或uniqueResult()方法返回查询的结果集

二、SQL查询

1、标量查询

标量查询会获得数据表列对应的Object数组组成的List，hibernate会默认通过ResultSetMetadata来判定所返回数据列的实际顺序和类型，但这样的默认处理会降低程序性能，因此在代码书写时建议明确返回值类型

　　　　　　  String sql = "select * from sql_student";
            SQLQuery q = session.createSQLQuery(sql)
                    .addScalar("id", StandardBasicTypes.LONG)//明确返回值类型，属性名称必须和表中列名相同
                    .addScalar("sname", StandardBasicTypes.STRING)
                    .addScalar("teacher_id", StandardBasicTypes.LONG);
            List list = q.list();
            for(Object ob : list) {
                Object[] ob1 = (Object[]) ob;
                System.out.println(ob1[0] + " | " + ob1[1] + " | " + ob1[2]);
            }

多表查询

String sql = "select t.tid, t.tname,s.sid,s.sname from sql_teacher t, sql_student s WHERE t.tid = s.teacher_id";
            SQLQuery q = session.createSQLQuery(sql)
                    .addScalar("t.tid", StandardBasicTypes.LONG)//明确返回值类型，属性名称必须和表中列名相同
                    .addScalar("t.tname", StandardBasicTypes.STRING)
                    .addScalar("s.sid", StandardBasicTypes.LONG)
                    .addScalar("s.sname", StandardBasicTypes.STRING);
            List list = q.list();
            for(Object ob : list) {
                Object[] ob1 = (Object[]) ob;
                System.out.println(ob1[0] + " " + ob1[1] + " | " + ob1[2] + " " + ob1[3]);
            }

2、实体查询

如果查询返回了某个数据表的全部数据列，且该数据表有对应的持久化类映射，就可用实体查询将查询结果转换成实体

　　　　　　  String sql = "select * from sql_student";
            SQLQuery q = session.createSQLQuery(sql)
                    .addEntity(SQLStudent.class);//程序必须选出所有数据列才可被转换成持久化实体
            List<SQLStudent> list = q.list();
            for(SQLStudent s : list) {
                System.out.println(s.getId() + " | " + s.getSname()+ " | " + s.getSqlTeacher().getName());
            }

多表查询(使用这种查询，如果两张表中有相同字段，则得到这两字段值均为顺序在前的字段的值，解决办法是将表中名称相同字段名称做区别)

　　　　　　  String sql = "SELECT s.*,t.*  FROM sql_teacher t,sql_student s WHERE s.teacher_id = t.tid";
            SQLQuery sqlQuery = session.createSQLQuery(sql)
                    .addEntity("t", SQLTeacher.class)
                    .addEntity("s", SQLStudent.class);
            List list = sqlQuery.list();
            for (Object obj : list) {
                Object[] objects = (Object[]) obj;
                SQLTeacher sqlTeacher = (SQLTeacher) objects[0];
                SQLStudent sqlStudent = (SQLStudent) objects[1];
                System.out.println(sqlTeacher.getId() + " " + sqlTeacher.getTname() + " | " + sqlStudent.getId() + " " + sqlStudent.getSname());
            }

3、关联查询

　　　　　　  String sql = "SELECT t.*,s.* FROM sql_teacher t LEFT JOIN sql_student s ON t.tid = s.teacher_id";
            SQLQuery sqlQuery = session.createSQLQuery(sql)
                    .addEntity("t", SQLTeacher.class)
                    .addEntity("s", SQLStudent.class);
            List list = sqlQuery.list();
            for (Object obj : list) {
                Object[] objects = (Object[]) obj;
                SQLTeacher sqlTeacher = (SQLTeacher) objects[0];
                SQLStudent sqlStudent = (SQLStudent) objects[1];
                System.out.println(sqlTeacher.getId() + " " + sqlTeacher.getTname() + " | " + sqlStudent.getId() + " " + sqlStudent.getSname());
            }

注意：两个表中字段名称不能重复，否则得到结果的相同名称字段的值会出现混淆（只取顺序排在前面的字段值）

 

测试实体类

@Entity
@Table(name = "sql_teacher")
public class SQLTeacher {
    @Id
    @GeneratedValue
    @Column(name = "tid")
    private Long id;
    @Column(name = "tname")
    private String tname;
    @OneToMany(targetEntity = SQLStudent.class, mappedBy = "sqlTeacher")
    private Set<SQLStudent> sqlStudents = new HashSet<SQLStudent>();

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getTname() {
        return tname;
    }

    public void setTname(String tname) {
        this.tname = tname;
    }

    public Set<SQLStudent> getSqlStudents() {
        return sqlStudents;
    }

    public void setSqlStudents(Set<SQLStudent> sqlStudents) {
        this.sqlStudents = sqlStudents;
    }
}

@Entity
@Table(name = "sql_student")
public class SQLStudent {
    @Id
    @GeneratedValue
    @Column(name = "sid")
    private Long id;
    @Column(name = "sname")
    private String sname;
    @ManyToOne(targetEntity = SQLTeacher.class)
    @JoinColumn(name = "teacher_id", referencedColumnName = "tid", nullable = false)
    private SQLTeacher sqlTeacher;
    //@Column(name = "teacher_id")
    //private Long teacherId;

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getSname() {
        return sname;
    }

    public void setSname(String sname) {
        this.sname = sname;
    }

    public SQLTeacher getSqlTeacher() {
        return sqlTeacher;
    }

    public void setSqlTeacher(SQLTeacher sqlTeacher) {
        this.sqlTeacher = sqlTeacher;
    }

    /*public Long getTeacherId() {
        return teacherId;
    }

    public void setTeacherId(Long teacherId) {
        this.teacherId = teacherId;
    }*/
}

测试类

public class SQLController {
    public static void main(String[] args) {
        Configuration cf = new Configuration().configure();
        SessionFactory sf = cf.buildSessionFactory();
        Session session = sf.openSession();
        Transaction ts = session.beginTransaction();
        try {
            SQLTeacher t = new SQLTeacher();
            t.setTname("teacher 3");
            Serializable id = session.save(t);
            t = (SQLTeacher) session.get(SQLTeacher.class, id);

            SQLStudent s1 = new SQLStudent();
            s1.setSname("student 1");
            s1.setSqlTeacher(t);

            SQLStudent s2 = new SQLStudent();
            s2.setSname("student 2");
            s2.setSqlTeacher(t);

            SQLStudent s3 = new SQLStudent();
            s3.setSname("student 3");
            s3.setSqlTeacher(t);

            session.save(s1);
            session.save(s2);
            session.save(s3);
            ts.commit();
        } finally {
            session.close();
            sf.close();
        }
    }
}

 代码下载：https://github.com/shaoyesun/hibernate_study.git