Mybatis的select查询的三种方式

1、首先建立一个测试的dao

 public interface IStudentDao {

     // 根据姓名查询
     List<Student> selectStudentsByName(String name);
 }

2、对这个dao进行实现

 public class StudentDaoImpl implements IStudentDao {

     private SqlSession sqlSession;

     // 根据姓名查询
     public List<Student> selectStudentsByName(String name) {
         List<Student> students = null;

         try {
             sqlSession = MybatisUtil.getSqlSession();
             students = sqlSession.selectList("selectStudentsByName", name);
         } catch (Exception e) {
             e.printStackTrace();
         } finally {
             if (sqlSession != null) {
                 sqlSession.close();
             }
         }
         return students;
     }
 }

3、utils的书写

 public class MybatisUtil {
     private static SqlSessionFactory sqlSessionFactory;

     public static SqlSession getSqlSession() {
         try {
             if (sqlSessionFactory == null) {
                 // 读取配置文件
                 InputStream inputStream = Resources
                         .getResourceAsStream("Mybatis.xml");
                 // 创建工厂
                 sqlSessionFactory = new SqlSessionFactoryBuilder()
                         .build(inputStream);
             }
         } catch (IOException e) {
             e.printStackTrace();
         }
         return sqlSessionFactory.openSession();
     }
 }

4、配置文档

（1）Mybatis.xml

 <?xml version="1.0" encoding="UTF-8" ?>
 <!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
 "http://mybatis.org/dtd/mybatis-3-config.dtd">
 <configuration>

     <properties resource="jdbc.properties" />

     <!-- 别名标签 -->
     <typeAliases>
         <typeAlias type="com.liuya.demo.mybaits.crud.pojo.Student"
             alias="Student" />
     </typeAliases>

     <!-- 配置运行的数据库环境 -->
     <environments default="mysqlenvironment">
         <environment id="mysqlenvironment">
             <!-- 連接池在本地连接中使用,在SSM中不用,用C3P0和DBCP -->
             <transactionManager type="JDBC" />
             <dataSource type="POOLED">
                 <property name="driver" value="${driver}" />
                 <property name="url" value="${url}" />
                 <property name="username" value="${username}" />
                 <property name="password" value="${password}" />
             </dataSource>
         </environment>
     </environments>

     <!-- 连接映射文件 -->
     <mappers>
         <!-- 最终使用的都是package -->
         <mapper resource="com/liuya/demo/mybaits/crud/mapper/StudentMapper.xml" />
     </mappers>
 </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

 public class MyTest {

     private IStudentDao idao;

     @Before
     public void before() {
         idao = new StudentDaoImpl();
     }
     // 根据name查询一个学生
     @Test
     public void testSelectStudentsByName() {
         System.out.println("开始查询学生");
         List<Student> students = idao.selectStudentsByName("张");
         for (Student student : students) {
             System.out.println(student);
         }
         System.out.println("查询学生成功");
     }
 }

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。