Mybatis的select查询的三种方式

Posted 2020-10-05 北极的大企鹅

1、首先建立一个测试的dao

1 public interface IStudentDao {
2 
3     // 根据姓名查询
4     List<Student> selectStudentsByName(String name);
5 }

2、对这个dao进行实现

 1 public class StudentDaoImpl implements IStudentDao {
 2 
 3     private SqlSession sqlSession;
 4 
 5     // 根据姓名查询
 6     public List<Student> selectStudentsByName(String name) {
 7         List<Student> students = null;
 8 
 9         try {
10             sqlSession = MybatisUtil.getSqlSession();
11             students = sqlSession.selectList("selectStudentsByName", name);
12         } catch (Exception e) {
13             e.printStackTrace();
14         } finally {
15             if (sqlSession != null) {
16                 sqlSession.close();
17             }
18         }
19         return students;
20     }
21 }

3、utils的书写

 1 public class MybatisUtil {
 2     private static SqlSessionFactory sqlSessionFactory;
 3 
 4     public static SqlSession getSqlSession() {
 5         try {
 6             if (sqlSessionFactory == null) {
 7                 // 读取配置文件
 8                 InputStream inputStream = Resources
 9                         .getResourceAsStream("Mybatis.xml");
10                 // 创建工厂
11                 sqlSessionFactory = new SqlSessionFactoryBuilder()
12                         .build(inputStream);
13             }
14         } catch (IOException e) {
15             e.printStackTrace();
16         }
17         return sqlSessionFactory.openSession();
18     }
19 }

4、配置文档

（1）Mybatis.xml

 1 <?xml version="1.0" encoding="UTF-8" ?>
 2 <!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
 3 "http://mybatis.org/dtd/mybatis-3-config.dtd">
 4 <configuration>
 5 
 6     <properties resource="jdbc.properties" />
 7 
 8     <!-- 别名标签 -->
 9     <typeAliases>
10         <typeAlias type="com.liuya.demo.mybaits.crud.pojo.Student"
11             alias="Student" />
12     </typeAliases>
13 
14     <!-- 配置运行的数据库环境 -->
15     <environments default="mysqlenvironment">
16         <environment id="mysqlenvironment">
17             <!-- 連接池在本地连接中使用,在SSM中不用,用C3P0和DBCP -->
18             <transactionManager type="JDBC" />
19             <dataSource type="POOLED">
20                 <property name="driver" value="${driver}" />
21                 <property name="url" value="${url}" />
22                 <property name="username" value="${username}" />
23                 <property name="password" value="${password}" />
24             </dataSource>
25         </environment>
26     </environments>
27 
28     <!-- 连接映射文件 -->
29     <mappers>
30         <!-- 最终使用的都是package -->
31         <mapper resource="com/liuya/demo/mybaits/crud/mapper/StudentMapper.xml" />
32     </mappers>
33 </configuration>

（2）properties的书写

#
##正式服务器
driver = com.mysql.jdbc.Driver
url = jdbc:mysql://127.0.0.1:3306/crud?useUnicode=true&characterEncoding=UTF-8&autoReconnect=true
username = root
password = 123456

（3）mapper的书写

<?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">

<!-- 配置数据库和实体类的字段 -->
<mapper namespace="test">

    <!-- 根据姓氏模糊查询 -->
    <select id="selectStudentsByName" resultType="Student">
        select
        NAME,AGE,SCORE from STUDENT where NAME like concat (‘%‘,#{ooo},‘%‘)
    </select>


</mapper>

5、测试的书写

 1 public class MyTest {
 2 
 3     private IStudentDao idao;
 4 
 5     @Before
 6     public void before() {
 7         idao = new StudentDaoImpl();
 8     }
 9     // 根据name查询一个学生
10     @Test
11     public void testSelectStudentsByName() {
12         System.out.println("开始查询学生");
13         List<Student> students = idao.selectStudentsByName("张");
14         for (Student student : students) {
15             System.out.println(student);
16         }
17         System.out.println("查询学生成功");
18     }
19 }

这就是一个完整的select查询的书写，重点在于mapper中的select书写

        写法一（比较复杂）：

<!-- 根据姓氏模糊查询 -->
    <select id="selectStudentsByName" resultType="Student">
        select
        NAME,AGE,SCORE from STUDENT where NAME like concat (‘%‘,#{ooo},‘%‘)
    </select>
  写法二（一般使用的比较多）：

<!-- 根据姓氏模糊查询 -->
    <select id="selectStudentsByName" resultType="Student">
        select
        NAME,AGE,SCORE from STUDENT where NAME like ‘%‘#{ooo}‘%‘
    </select>
写法三（不建议使用，会有sql注入问题和加载速度慢的问题）：

<!-- 根据姓氏模糊查询 -->
    <select id="selectStudentsByName" resultType="Student">
        select
        NAME,AGE,SCORE from STUDENT where NAME like ‘%&{value}%‘
    </select>
like后的括号值，只能是value。