3. 高级映射
前期准备：
- 数据库表
-- ----------------------------
-- Table structure for student
-- ----------------------------
DROP TABLE IF EXISTS `student`;
CREATE TABLE `student` (
`sid` int(0) NOT NULL AUTO_INCREMENT,
`sname` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL,
`tid` int(0) NULL DEFAULT NULL,
PRIMARY KEY (`sid`) USING BTREE
) ENGINE = InnoDB CHARACTER SET = utf8 COLLATE = utf8_general_ci ROW_FORMAT = Dynamic;
-- ----------------------------
-- Records of student
-- ----------------------------
INSERT INTO `student` VALUES (1, '小红', 1);
INSERT INTO `student` VALUES (2, '小明', 1);
INSERT INTO `student` VALUES (3, '小王', 1);
INSERT INTO `student` VALUES (4, '小赵', 1);
SET FOREIGN_KEY_CHECKS = 1;
====================================================>
-- ----------------------------
-- Table structure for teacher
-- ----------------------------
DROP TABLE IF EXISTS `teacher`;
CREATE TABLE `teacher` (
`tid` int(0) NOT NULL,
`tname` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL,
PRIMARY KEY (`tid`) USING BTREE
) ENGINE = InnoDB CHARACTER SET = utf8 COLLATE = utf8_general_ci ROW_FORMAT = Dynamic;
-- ----------------------------
-- Records of teacher
-- ----------------------------
INSERT INTO `teacher` VALUES (1, 'A老师');
SET FOREIGN_KEY_CHECKS = 1;
-
数据库结构图
image.png
image.png
3.1 多对一
多对一的意思是指多个学生对应一个老师，有可能是一个实体类A对象里面有另外一个实体类B对象，但是数据库查询的时候并没有B对象的属性的情况使用resultMap进行高级映射
示例：查询学生和老师姓名的对应关系
3.1.1 创建实体类
- 学生对象包含老师对象
package pojo;
public class Student {
private int id;
private String name;
//老师对象存在
private Teacher teacher;
//getter&setter省路
}
- 教师对象
package pojo;
import java.util.List;
public class Teacher {
private int id;
private String name;
3.1.2 创建查询SQL语句
可以看到查询语句中对应的Java实体类Student对象中并没有tname的字段，只有teacher对象，这种情况下就需要用resultMap来进行映射
SELECT s.sid,s.sname,t.tname FROM student s INNER JOIN teacher t ON s.tid = t.tid
3.1.3 创建StudentMapper.xml
-
property：在Java程序中的字段 -
column：在数据库中的字段 -
type：返回的类型
3.1.4 创建StudentMapper接口
package dao;
import pojo.Student;
import pojo.Teacher;
import java.util.List;
public interface StudentMapper {
List getStudent();
}
3.1.5 创建测试类
public class MybatisTest {
public static void main(String[] args) throws Exception{
SqlSession sqlSession = MybatisUtils.getSqlSession();
UserMapper mapper = sqlSession.getMapper(UserMapper.class);
List students = mapper.getStudent();
for (Student student:students) {
System.out.println(student);
}
}
}
3.2 多对一
和一对多情况类似，就是一个老师对应多个学生，创建教师实体类时包含了一个集合对象
3.2.1 创建实体类对象
- 包含学生集合的情况
package pojo;
import java.util.List;
public class Teacher {
private int id;
private String name;
private List students;
// getter&setter
}
- 学生实体类
package pojo;
public class Student {
private int id;
private String name;
private int tid;
// getter&setter
}
3.2.2 创建查询语句
老师的实体类中并没有学生对象的属性只有集合，所以需要resultMap对集合进行映射
SELECT t.tid,t.tname,s.sid,s.sname FROM student s INNER JOIN teacher t ON s.tid = t.tid
3.2.3 创建TeacherMapper.xml
-
ofType：要查出集合元素的类型
3.2.4 创建TeacherMapper接口
package dao;
import pojo.Student;
import pojo.Teacher;
import java.util.List;
public interface UserMapper {
List getTeacher();
}
3.2.5 创建测试类
public class MybatisTest {
public static void main(String[] args) throws Exception{
SqlSession sqlSession = MybatisUtils.getSqlSession();
UserMapper mapper = sqlSession.getMapper(UserMapper.class);
List teachers = mapper.getTeacher();
for (Teacher teacher:teachers) {
System.out.println(teacher);
}
}
}
4. 动态SQL
动态SQL就是根据不同的条件进行SQL的拼接，实现动态SQL需要使用标签跟JSTL类似
- if
- choose (when, otherwise)
- trim (where, set)
- foreach
4.1 数据库环境准备
image.png
4.2 if标签
test=""内部写判断条件即可
代码示例
DAO层
public interface BlogMapper {
List queryBlog1(Map map);
}
测试类
public class MybatisTest {
public static void main(String[] args) throws Exception{
SqlSession sqlSession = MybatisUtils.getSqlSession();
BlogMapper mapper = sqlSession.getMapper(BlogMapper.class);
Map map1 = new HashMap<>();
map1.put("title","Java如此简单");
List blogs = mapper.queryBlog1(map1);
for (Blog blog:blogs) {
System.out.println(blog);
}
sqlSession.close();
}
}
效果展示
image.png
补充
可以把1=1 换成
4.2 choose标签
-
-
4.3 set标签
update blog
title = #{title}
author = #{author}
where id = #{id}
4.4 foreach标签
TODO有用到再更新