mybatis通过if语句实现增删改查操作

有时候为了简化我们的代码。

1 举个例子

Student类：

@Data
public class Student {
 private Integer id;
 private Integer age;
 private Integer sno;
}

有时候我们想通过age这个属性获取Student对象

有时候我们也想通过sno这个属性获取Student对象

难道我们在DAO层写两个接口？

比如这样子？

Student getStudentByAge(Int age);

Student getStudentBySno(Int sno);

那么在mapper文件中要这样写？

 <select id="getStudentByAge" parameterType="int" resultMap="studentMap">
 select * from student where age=#{age}
 </select>
 <select id="getStudentBySno" parameterType="int" resultMap="studentMap">
 select * from student where sno=#{sno}
 </select>

显然，这样子是不高效的

2 上手测试实验

实体类 Student：

@Data
public class Student {
 @ApiModelProperty(name = "id",example = "1",position = 1)
 private Integer id;
 @ApiModelProperty(name = "age",value = "年龄",example = "18",position = 2)
 private Integer age;
 @ApiModelProperty(name = "sno",value = "学号",example = "334",position = 3)
 private Integer sno;
}

数据库：

CREATE TABLE `student` (
 `id` int(11) NOT NULL AUTO_INCREMENT,
 `age` int(11) DEFAULT NULL COMMENT '年龄',
 `sno` int(11) NOT NULL COMMENT '学号',
 PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8;

手动添加一些数据

Dao层：

@Mapper
public interface StudentDao {

 /**
 * @description: 通过student中的属性 查询到student
 * @param: student
 * @author: Yuz
 * @creat_time: 2019/8/20
 * @return: student
 **/
 Student getStudent(Student student);

 /**
 * @description: 通过age sno 属性来删除
 * @param: student
 * @author: Yuz
 * @creat_time: 2019/8/20
 * @return: void
 **/
 void deleteStudent(Student student);
}

Mapper

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd" >
<mapper namespace="com.dao.StudentDao">
 <resultMap id="studentMap" type="com.entity.Student">
 <id property="id" column="id"/>
 <result property="age" column="age"/>
 <result property="sno" column="sno"/>
 </resultMap>

 <select id="getStudent" parameterType="com.entity.Student" resultMap="studentMap">
 select * from student where
 <if test="age != null">age=#{age}</if>
 <if test="sno !=null">sno=#{sno}</if>
 </select>

 <delete id="deleteStudent" parameterType="Student">
 delete from student
 <where>
  <if test="age != null">
  age =#{age}
  </if>
  <if test="sno != sno">
  sno=#{sno}
  </if>
 </where>
 </delete>

</mapper>

Service层：

@Service
public class StudentService {

 @Autowired
 StudentDao studentDao;

 /**
 * @description: 通过student中的属性 查询到student
 * @param: student
 * @author: Yuz
 * @creat_time: 2019/8/20
 * @return: student
 **/
 public Student getStudent(Student student){
 return studentDao.getStudent(student);
 }

 /**
 * @description: 通过age sno 属性来删除
 * @param: student
 * @author: Yuz
 * @creat_time: 2019/8/20
 * @return: void
 **/
 public void deleteStudent(Student student){
 studentDao.deleteStudent(student);
 }
}

Controller:

@RestController
@Api("学生接口")
@RequestMapping("/student")
public class StudentController {
 @Autowired
 StudentService studentService;

 /**
 * @description: 通过student中的属性 查询到student
 * @param: student
 * @author: Yuz
 * @creat_time: 2019/8/20
 * @return: student
 **/
 @ApiOperation("通过属性查询student")
 @PostMapping("/getStudent")
 Student getStudent(@RequestBody Student student){
 return studentService.getStudent(student);
 }

 /**
 * @description: 通过age sno 属性来删除
 * @param: student
 * @author: Yuz
 * @creat_time: 2019/8/20
 * @return: void
 **/
 @ApiOperation("通过属性删除student")
 @PostMapping("/delete")
 public void deleteStudent(@RequestBody Student student){
 studentService.deleteStudent(student);
 }
}

3 直接测试

通过age属性查询student：成功

通过sno属性查询:

通过属性age删除Student:

通过sno属性删除Student

补充知识：mybatis使用if条件判断，数字类型不能写 0 ！=‘'，否则会进不到条件拼接里面

1.对于 if条件判断：数字类型属性判断的时候

注意不可以是这种情况

<if test="delFlag!= null and delFlag!= ''">
 and del_flag = #{delFlag}
</if>

参数一个是0，一个是""，最终debug会走进case 8 里面，0和“”都会被转成double进行比较，都会变成0.0，这就是mybati中if test 0!=""判定为false的原因

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