MyBatis知识点总结-DX的笔记
MyBatis知识点总结
什么是MyBatis?
- 它是一款半自动的ORM持久层框架，具有较高的SQL灵活性，支持高级映射(一对一，一对多)，动态SQL，延迟加载和缓存等特性，但它的数据库无关性较低
- ORM（Object Relation Mapping）对象关系映射
- 对象：Java对象
- 关系：数据库中的关系模型
- 对象关系映射：Java对象和数据库的关系模型之间建立一种对用关系
- 比如用一个Java的Student类，去对应数据库中的一张student表，类中的属性和表中的列一一对应
- Student类就对应student表，一个Student对象就对应student表中的一行数据
- mybatis的特点
- mybatis是：半自动的ORM框架，需要手动编写SQL语句
- 全自动的ORM框架（如：hibernate），不需要编写SQL语句
- hibernate只需设置好ORM映射关系，就可以实现CRUD操作
- mybatis需要手写SQL，灵活性更高，但是换数据库时，需要重写SQL，因为mybatis的数据无关性低
- 相对于JDBC，它提供了输入映射和输出映射，方便sql的参数设置和结果集封装
- 提供了关联查询和动态SQL等功能，极大地提升了开发的效率
项目创建思路整理
项目结构
-
创建Maven项目
-
在pom.xml文件中导包
<dependencies> <dependency> <groupId>junitgroupId> <artifactId>junitartifactId> <version>4.11version> <scope>testscope> dependency> <dependency> <groupId>org.mybatisgroupId> <artifactId>mybatisartifactId> <version>3.5.1version> dependency> <dependency> <groupId>mysqlgroupId> <artifactId>mysql-connector-javaartifactId> <version>8.0.19version> dependency> dependencies> -
在Java目录下建包
- entity 实体类
- utils 工具类
- dao 数据操作对象
-
在resources目录下建mybatis-config.xml文件
项目搭建好，开始干活
-
编写实体类
- Student.java
- 成员变量与字段名尽量相同
-
连接数据库的准备
-
在mybatis-config.xml中写配置信息
DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN" "http://mybatis.org/dtd/mybatis-3-config.dtd"> <configuration> <settings> <setting name="logImpl" value="STDOUT_LOGGING"/> settings> <environments default="mysql"> <environment id="mysql"> <transactionManager type="JDBC">transactionManager> <dataSource type="POOLED"> <property name="driver" value="com.mysql.cj.jdbc.Driver"/> <property name="url" value="jdbc:mysql:///stusys?serverTimezone=UTC"/> <property name="username" value="root"/> <property name="password" value="111111"/> dataSource> environment> environments> <mappers> <mapper resource="mapper/StudentMapper.xml">mapper> mappers> configuration>
-
-
写数据库连接的工具类
public class MyBatisUtil { static SqlSessionFactory factory=null; static { try { InputStream in = Resources.getResourceAsStream("mybatis-config.xml"); factory = new SqlSessionFactoryBuilder().build(in); } catch (IOException e) { e.printStackTrace(); } } public static SqlSession getSqlSession(){ return factory.openSession(); } } -
开始写DAO层（数据连接对象）
-
写StudentDao接口文件
public interface StudentDao { List<Student> select(); Student selectById(int id); int insert(Student student); int updata(Student student); int deleteById(int id); } -
写映射文件StudentMapper.xml
DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd"> <mapper namespace="StudentMapper"> <select id="select" resultType="com.dx.entity.Student"> select * from student select> <select id="selectById" parameterType="int" resultType="com.dx.entity.Student"> select * from student where id=#{id} select> <insert id="insert" parameterType="com.dx.entity.Student"> insert into student(sname,age,gender,address,phone,remark) values(#{sname},#{age},#{gender},#{address},#{phone},#{remark}) insert> <update id="updata" parameterType="com.dx.entity.Student"> update student set sname=#{sname},age=#{age},gender=#{gender},address=#{address},phone=#{phone},remark=#{remark} where id=#{id} update> <delete id="deleteById"> delete from student where id = #{id} delete> mapper>
-
-
写实现类
public class StudentDaoImpl implements StudentDao { @Override public List<Student> select() { SqlSession sqlSession = MyBatisUtil.getSqlSession(); //传入的第一个参数为：mapper文件中的命名空间名称.sql语句id List<Student> students = sqlSession.selectList("StudentMapper.select"); sqlSession.close(); return students; } @Override public Student selectById(int id) { SqlSession sqlSession = MyBatisUtil.getSqlSession(); //传入的第二个参数为：查询条件，如果有多个查询条件应该封装成对象(对象的成员变量名要与sql的字段名保持一致)或使用map Student student = sqlSession.selectOne("StudentMapper.selectById",id); sqlSession.close(); return student; } @Override public int insert(Student student) { SqlSession sqlSession = MyBatisUtil.getSqlSession(); int result = sqlSession.insert("StudentMapper.insert", student); //增、删、改 都需要提交事务 //jdbc默认自动提交，MyBatis默认手动提交 sqlSession.commit(); sqlSession.close(); return result; } @Override public int updata(Student student) { SqlSession sqlSession = MyBatisUtil.getSqlSession(); int result = sqlSession.insert("StudentMapper.updata", student); //增、删、改 都需要提交事务 //jdbc默认自动提交，MyBatis默认手动提交 sqlSession.commit(); sqlSession.close(); return result; } @Override public int deleteById(int id) { SqlSession sqlSession = MyBatisUtil.getSqlSession(); int result = sqlSession.insert("StudentMapper.deleteById", id); //增、删、改 都需要提交事务 //jdbc默认自动提交，MyBatis默认手动提交 sqlSession.commit(); sqlSession.close(); return result; } } -
写测试类
public class MyTest { @Test public void crudTest(){ StudentDaoImpl studentDao = new StudentDaoImpl(); //查 // Liststudent = new StudentDaoImpl().select(); // Student student = studentDao.selectById(2); // System.out.println(student); //增 // Student student = new Student("莎莎", 22, "1", "zhegnzhou", "1242330", "iafg"); // int res = studentDao.insert(student); //改 // Student student = new Student(17, "shasha", 22, "1", "zhegnzhou", "1242330", "iafg"); // int res = studentDao.updata(student); //删 int res = studentDao.deleteById(17); if (res == 1) { System.out.println("成功"); } else { System.out.println("失败"); } } }
代理模式
dao接口+mapper文件
要求
- mapper文件中的命名空间必须与dao接口的全限定名保持一致
- mapper文件中的sql语句ID必须与dao接口中的方法名称保持一致
- mapper文件中parameterType传入参数类型必须与dao接口中的方法形参类型保持一致
- mapper文件中resultType传出结果类型必须与dao接口中的方法返回值“保持一致”
使用方法
-
不使用代理模式
SqlSession sqlSession = MybatisUtil.getSqlSession(); //两个参数（Mapper的命名空间名.sql标签的id , [传入的参数] ） Emp emp = sqlSession.selectOne("empNamespace.selectById", 7566); System.out.println(emp); sqlSession.close(); -
使用代理
SqlSession sqlSession = MybatisUtil.getSqlSession(); //获取Mapper代理对象 getMapper(接口.class) 返回值为当前指定接口的实现对象 DeptDao deptDao = sqlSession.getMapper(DeptDao.class); Dept dept = deptDao.selectById(10); System.out.println(dept); sqlSession.close();
mybatis配置文件
全局设置（日志和下划线转驼峰）
<settings>
<setting name="mapUnderscoreToCamelCase" value="true"/>
<setting name="logImpl" value="LOG4J"/>
settings>
设置别名
- 自带的
- int -> java.lang.Integer
- _int -> int
- double
- string->java.lang.String
- map -> java.util.Map,
- hashmap -> java.util.HashMap
- list -> java.util.List
- 自定义别名
<typeAliases>
<typeAlias type="com.dx.entity.Emp" alias="emp"/>
<package name="com.dx.entity"/>
typeAliases>
配置数据操作环境
<environments default="mysql">
<environment id="mysql">
<transactionManager type="JDBC">transactionManager>
<dataSource type="POOLED">
<property name="driver" value="com.mysql.cj.jdbc.Driver"/>
<property name="url" value="jdbc:mysql:///student?serverTimezone=UTC"/>
<property name="username" value="root"/>
<property name="password" value="111111"/>
dataSource>
environment>
<environment id="oracle">
<transactionManager type="JDBC">transactionManager>
<dataSource type="POOLED">
<property name="driver" value="oracle.jdbc.driver.OracleDriver"/>
<property name="url" value="jdbc:oracle:thin:localhost:1521:orcl"/>
<property name="username" value="root"/>
<property name="password" value="111111"/>
dataSource>
environment>
environments>
加载mapper映射文件
<mappers>
<mapper resource="mapper/EmpMapper.xml"/>
<mapper class="com.bjpowernode.dao.EmpDao"/>
<package name="com.bjpowernode.dao"/>
mappers>
log4j日志管理
日志配置文件
# 全局日志配置
log4j.rootLogger=DEBUG, stdout
# MyBatis 日志配置
#局部日志记录配置
log4j.logger.org.mybatis.example.BlogMapper=TRACE
# 控制台输出
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%d %p %c.%M() --%m%n
### 输出DEBUG 级别以上的日志到=E://logs/error.log ###
log4j.appender.file = org.apache.log4j.DailyRollingFileAppender
log4j.appender.file.File = D:/logs/test.log
#log4j.appender.file.Append = true
#log4j.appender.file.Threshold = DEBUG
log4j.appender.file.layout = org.apache.log4j.PatternLayout
log4j.appender.file.layout.ConversionPattern = %-d{yyyy-MM-dd HH:mm:ss} [ %t:%r ] - [ %p ] %m%n
### 输出DEBUG 级别以上的日志到=E://logs/error.log ###
log4j.appender.E = org.apache.log4j.DailyRollingFileAppender
log4j.appender.E.File =E://logs/error.log
log4j.appender.E.Append = true
log4j.appender.E.Threshold = ERROR
log4j.appender.E.layout = org.apache.log4j.PatternLayout
log4j.appender.E.layout.ConversionPattern = %-d{yyyy-MM-dd HH:mm:ss} [ %t:%r ] - [ %p ] %m%n
log4j的使用
/**
* java.util.logging.Logger(JUL)
*/
private Logger logger = Logger.getLogger(Log4jTest.class);
@Test
public void run(){
//错误信息
logger.error("error日志");
//警告信息
logger.warn("warn日志");
//普通信息
logger.info("info日志");
//调试信息
logger.debug("debug日志");
//底层信息
logger.trace("trace日志");
}
映射配置文件
结果映射
-
自动映射
<select id="selectAll" resultType="com.dx.entity.Emp"> select empno,ename,job,mgr,hiredate,sal,comm,deptno from emp order by empno desc select> -
手动映射
<resultMap id="ResultMap" type="emp"> <id column="empno" property="empno"/> <result column="ename" property="ename"/> <result column="job" property="job"/> <result column="mgr" property="mgr"/> <result column="hiredate" property="hiredate"/> <result column="sal" property="sal"/> <result column="comm" property="comm"/> <result column="deptno" property="deptno"/> resultMap> <select id="selectAll2" resultMap="ResultMap"> select empno,ename,job,mgr,hiredate,sal,comm,deptno from emp order by empno desc select>
占位符#与$
-
${}的sql是静态sql
-
#{}的sql是动态sql
-
<select id="selectById" parameterType="int" resultType="Emp"> select empno,ename,job,mgr,hiredate,sal,comm,deptno from emp where empno=#{empno} select> <select id="selectById2" parameterType="int" resultType="Emp"> select empno,ename,job,mgr,hiredate,sal,comm,deptno from emp where empno=${value} select>
模糊查询
<select id="selectByEname" parameterType="String" resultType="emp">
select empno,ename,job,mgr,hiredate,sal,comm,deptno from emp where ename like concat('%',#{name},'%')
select>
<select id="selectByEname2" parameterType="String" resultType="emp">
select empno,ename,job,mgr,hiredate,sal,comm,deptno from emp where ename like '%${value}%'
select>
<select id="selectByEname3" parameterType="String" resultType="Emp">
<bind name="keyname" value="'%'+_parameter+'%'"/>
select empno,ename,job,mgr,hiredate,sal,comm,deptno from emp where ename like #{keyname}
select>
传入多参数
建议：参数为2-4个使用@Param，参数过多使用实体类或Map集合
传入参数为Map
-
mapper.xml
<select id="selectByParam" parameterType="map" resultType="emp"> select empno,ename,job,mgr,hiredate,sal,comm,deptno from emp where deptno=#{aaa} and sal>=#{bbb} select> -
test.java
@Test public void selectByMap(){ SqlSession sqlSession = MybatisUtils.getSqlSession(); EmpDao empDao = sqlSession.getMapper(EmpDao.class); HashMap<String, Object> map = new HashMap<>(); map.put("aaa",10); map.put("bbb",2000); List<Emp> emps = empDao.selectByParam(map); for (Emp emp : emps) { System.out.println(emp); } sqlSession.close(); }
使用@Param注解
-
mapper.xml
<select id="selectByParam2" parameterType="map" resultType="emp"> select empno,ename,job,mgr,hiredate,sal,comm,deptno from emp where deptno=#{deptno} and sal>=#{sal} select> -
dao.java接口
public List<Emp> selectByParam2(@Param(value="deptno") Integer deptno,@Param("sal") Double sal); -
test.java
@Test public void selectByMap2(){ SqlSession sqlSession = MybatisUtils.getSqlSession(); EmpDao empDao = sqlSession.getMapper(EmpDao.class); List<Emp> emps = empDao.selectByParam2(10,3000.0); for (Emp emp : emps) { System.out.println(emp); } sqlSession.close(); }
分页查询
-
使用sql语句
<select id="selectPage" parameterType="map" resultType="emp"> select empno,ename,job,mgr,hiredate,sal,comm,deptno from emp order by empno limit #{page},#{num} select> -
使用 RowBounds()
<select id="selectPage2" resultType="emp"> select empno,ename,job,mgr,hiredate,sal,comm,deptno from emp order by empno select>public List<Emp> selectPage2(RowBounds rowBounds);//使用RowBounds(起始坐标，展示条数)实现 RowBounds rowBounds = new RowBounds(10, 5); List<Emp> emps = empDao.selectPage2(rowBounds); -
使用PageHelper插件
<dependency> <groupId>com.github.pagehelpergroupId> <artifactId>pagehelperartifactId> <version>5.2.0version> dependency><plugins> <plugin interceptor="com.github.pagehelper.PageInterceptor">plugin> plugins><select id="selectPage3" resultType="emp"> select empno,ename,job,mgr,hiredate,sal,comm,deptno from emp order by empno select>//使用分页插件 PageHelper.startPage(当前页码,每页条数); PageHelper.startPage(1,5); List<Emp> emps = empDao.selectPage3(); //在PageHelper提供一个PageInfo的分页工具类 PageInfo<Emp> pageInfo = new PageInfo<>(list); pageInfo.getList(); System.out.println("当前页码：" + pageInfo.getPageNum()); System.out.println("每页条数：" + pageInfo.getPageSize()); System.out.println("总记录数：" + pageInfo.getTotal()); System.out.println("总页数：" + pageInfo.getPages()); System.out.println("上一页：" + pageInfo.getPrePage()); System.out.println("下一页：" + pageInfo.getNextPage()); System.out.println("是否有上一页：" + pageInfo.isHasPreviousPage()); System.out.println("是否有下一页：" + pageInfo.isHasNextPage()); System.out.println("是否为首页：" + pageInfo.isIsFirstPage()); System.out.println("是否为末页：" + pageInfo.isIsLastPage()); System.out.println("页码数组：" + Arrays.toString(pageInfo.getNavigatepageNums()));
插入数时返回主键值
-
<insert id="insert" parameterType="emp" useGeneratedKeys="true" keyColumn="empno" keyProperty="empno"> insert into emp(ename,job,mgr,hiredate,sal,comm,deptno) values (#{ename},#{job},#{mgr},#{hiredate},#{sal},#{comm},#{deptno}) insert> -
@Test public void insertTest() { SqlSession sqlSession = MybatisUtils.getSqlSession(); EmpDao empDao = sqlSession.getMapper(EmpDao.class); Emp emp = new Emp("小小", "经理", 100, new Date(), 5600.0, 1000.0, 20); System.out.println("数据插入前empno="+emp.getEmpno()); empDao.insert(emp); System.out.println("数据插入后empno="+emp.getEmpno()); sqlSession.commit(); sqlSession.close(); }
动态sql
-
if
<select id="selectUseIf" resultType="emp"> select empno,ename,job,mgr,hiredate,sal,comm,deptno from emp where <if test="ename!=null and ename !='' "> ename like concat('%',#{ename},'%') if> <if test="sal!=null"> and sal >= #{sal} if> select> <select id="selectUseIf2" parameterType="String" resultType="emp"> select empno,ename,job,mgr,hiredate,sal,comm,deptno from emp where <if test="_parameter!=null and _parameter !='' "> ename = #{ename} if> select> -
where和choose
<select id="selectUseWhere" resultType="emp"> select empno,ename,job,mgr,hiredate,sal,comm,deptno from emp <where> <if test="ename!=null and ename !='' "> ename like concat('%',#{ename},'%') if> <if test="sal!=null"> and sal >= #{sal} if> where> select> -
set
<update id="updateUseSet" parameterType="emp"> update emp <set> <if test="ename!=null"> ename=#{ename}, if> <if test="job!=null"> job=#{job}, if> set> where empno=#{empno} update> -
foreach
<delete id="deleteUseForeach" parameterType="int"> delete from emp where empno in <foreach collection="array" open="(" close=")" separator="," item="id"> #{id} foreach> delete> <delete id="deleteUseForeach2" parameterType="int"> delete from emp where empno in <foreach collection="list" open="(" close=")" separator="," item="id"> #{id} foreach> delete> -
Trim
<insert id="insertUseTrim" parameterType="com.dx.entity.Emp"> insert into emp <trim prefix="(" suffix=")" suffixOverrides=","> <if test="ename!=null"> ename, if> <if test="job!=null"> job, if> <if test="mgr!=null"> mgr, if> <if test="hiredate!=null"> hiredate, if> <if test="sal!=null"> sal, if> <if test="comm!=null"> comm, if> <if test="deptno!=null"> deptno, if> trim> <trim prefix=" values (" suffix=")" suffixOverrides=","> <if test="ename!=null"> #{ename}, if> <if test="job!=null"> #{job}, if> <if test="mgr!=null"> #{mgr}, if> <if test="hiredate!=null"> #{hiredate}, if> <if test="sal!=null"> #{sal}, if> <if test="comm!=null"> #{comm}, if> <if test="deptno!=null"> #{deptno} if> trim> insert> -
测试类
@Test public void if_where_choose_Test(){ SqlSession sqlSession = MybatisUtils.getSqlSession(); EmpDao empDao = sqlSession.getMapper(EmpDao.class); Emp param = new Emp(); param.setEname("S"); param.setSal(1000.0); List<Emp> empList = empDao.selectUseIf(param); List<Emp> empList = empDao.selectUseWhere(param); for (Emp emp : empList) { System.out.println(emp); } sqlSession.close(); } @Test public void set_Test(){ SqlSession sqlSession = MybatisUtils.getSqlSession(); EmpDao empDao = sqlSession.getMapper(EmpDao.class); Emp param = new Emp(); param.setEmpno(666); param.setEname("shasha"); param.setJob("医生"); empDao.updateUseSet(param); System.out.println("插入成功！"); sqlSession.commit(); sqlSession.close(); } @Test public void foreachTest(){ SqlSession sqlSession = MybatisUtils.getSqlSession(); EmpDao empDao = sqlSession.getMapper(EmpDao.class); //批量删除-数组 empDao.deleteUseForeach(new Integer[]{1,2,3,4}); //批量删除-集合 empDao.deleteUseForeach2(Arrays.asList(1,2,3,4,5,6)); sqlSession.commit(); System.out.println("删除成功"); sqlSession.close(); }
使用注解
使用lombok
-
导包
<dependency> <groupId>org.projectlombokgroupId> <artifactId>lombokartifactId> <version>1.18.24version> dependency> -
常用注解
- @Setter 注解在类或字段，注解在类时为所有字段生成setter方法，注解在字段上时只为该字段生成setter方法。
- @Getter 使用方法同上，区别在于生成的是getter方法。
- @ToString 注解在类，添加toString方法。
- @EqualsAndHashCode 注解在类，生成hashCode和equals方法。
- @NoArgsConstructor 注解在类，生成无参的构造方法。
- @RequiredArgsConstructor 注解在类，为类中需要特殊处理的字段生成构造方法，比如final和被@NonNull注解的字段。
- @AllArgsConstructor 注解在类，生成包含类中所有字段的构造方法。
- @Data 注解在类，生成setter/getter、equals、canEqual、hashCode、toString方法，如为final属性，则不会为该属性生成setter方法。
- @Slf4j 注解在类，生成log变量，严格意义来说是常量。private static final Logger log = LoggerFactory.getLogger(UserController.class);
Mybatis注解
-
@Select
-
@Insert
-
@Delete
-
@Update
-
简单的sql可以用，复杂的不推荐使用
-
用法
public interface UserMapper { @Insert("insert into user (name,pwd) value (#{name},#{pwd})") Integer insert(User user); @Delete("delete from user where id = #{id}") Integer delete(Integer id); @Select("select id,name,pwd from user ") List<User> selectAll(); /* 动态sql 写法 */ @Select("") List<User> selectList(@Param("age") Integer age); }
缓存
一级缓存
-
一级缓存：在同一个sqlSession中
-
一级缓存是内存式缓存，必须使用，不能关闭
-
查询时，先查询一级缓存，如果没有找到数据再发送sql语句查询数据，并将结果放到sqlSession中
-
@Test public void firstCacheTest(){ SqlSession sqlSession = MybatisUtils.getSqlSession(); System.out.println("——————————————————————————第一次查询——————————————————————————"); UserDao userDao = sqlSession.getMapper(UserDao.class); User user = userDao.selectUserById(2); System.out.println(user); //清空一级缓存 //sqlSession.commit(); System.out.println("——————————————————————————第二次查询——————————————————————————"); UserDao userDao2 = sqlSession.getMapper(UserDao.class); User user2 = userDao2.selectUserById(2); System.out.println(user2); System.out.println("——————————————————————————两次结果对比——————————————————————————"); System.out.println(user==user2); sqlSession.close(); }
二级缓存
-
二级缓存：在同一个SqlSessionFactory中
-
二级缓存有三个开关
- 第一个在mybatis-config.xml中设置cacheEnabled为true（默认开启）
- 第二个在mapper.xml中加上 (不加上就相当于关闭)
- 第三个在select标签中配置 useCache=“true” （默认开启）
-
被缓存的数据需要支持序列化,所以存储的对象必须实现可序列化接口
-
@Test public void secondCacheTest(){ System.out.println("——————————————————————————第一次查询——————————————————————————"); SqlSession sqlSession = MybatisUtils.getSqlSession(); UserDao userDao = sqlSession.getMapper(UserDao.class); User user = userDao.selectUserById(2); System.out.println(user); sqlSession.close(); System.out.println("——————————————————————————第二次查询——————————————————————————"); SqlSession sqlSession2 = MybatisUtils.getSqlSession(); UserDao userDao2 = sqlSession2.getMapper(UserDao.class); User user2 = userDao2.selectUserById(2); System.out.println(user2); sqlSession2.close(); System.out.println("——————————————————————————两次结果对比——————————————————————————"); System.out.println(user==user2); }
联表查询⭐
使用的数据库
- student表
| id | sname | cid |
|---|---|---|
| 1 | 小李 | 1 |
| 2 | 小陈 | 2 |
| 3 | 小郭 | 2 |
| 4 | 小夏 | 1 |
- classes表
| id | cname |
|---|---|
| 1 | 一班 |
| 2 | 二班 |
| 3 | 三班 |
一对一
-
查询学生和其所在的班级
-
//Dao接口 List<Student> selectStudentAndClasses(); -
<resultMap id="selectResultMap" type="com.dx.entity.Student"> <id column="id" property="id"/> <result column="sname" property="sname"/> <result column="cid" property="cid"/> <association property="classes" javaType="com.dx.entity.Classes"> <id column="id" property="id"/> <result column="cname" property="cname"/> association> resultMap> <select id="selectStudentAndClasses" resultMap="selectResultMap"> select s.id,s.sname,s.cid,c.cname from student s join classes c on s.cid=c.id select> -
扩展：使用 代码块 和 继承
<resultMap id="BaseResultMap" type="com.dx.entity.Student"> <id column="id" property="id"/> <result column="sname" property="sname"/> <result column="cid" property="cid"/> resultMap> <resultMap id="selectResultMap2" type="com.dx.entity.Student" extends="BaseResultMap"> <association property="classes" javaType="com.dx.entity.Classes"> <id column="id" property="id"/> <result column="cname" property="cname"/> association> resultMap> <sql id="FeildList"> s.id,s.sname,s.cid,c.cname sql> <select id="selectStudentAndClasses2" resultMap="selectResultMap"> select <include refid="FeildList">include> from student s join classes c on s.cid=c.id select>
一对多
-
查询某个班级里的学生
-
Classes selectById(Integer id); -
<resultMap id="BaseResultMap" type="com.dx.entity.Classes"> <id column="id" property="id"/> <result column="cname" property="cname"/> resultMap> <resultMap id="selectByIdResultMap" type="com.dx.entity.Classes" extends="BaseResultMap"> <collection property="studentList" ofType="com.dx.entity.Student"> <id column="sid" property="id"/> <result column="sname" property="sname"/> collection> resultMap> <select id="selectById" resultMap="selectByIdResultMap"> select c.id,c.cname,s.id sid,s.sname from classes c left join student s on c.id=s.cid where c.id=#{id} select>
关联查询
-
Dao接口
Classes selectOne(Integer id); -
StudentMapper.xml
<select id="selectByCid" resultType="com.dx.entity.Student"> select id,sname from student where cid=#{cid} select> -
ClassesMapper.xml
<resultMap id="SelectOneResultMap" type="com.dx.entity.Classes" extends="BaseResultMap"> <collection property="studentList" ofType="com.dx.entity.Student" column="id" select="com.dx.dao.StudentDao.selectByCid"> collection> resultMap> <select id="selectOne" parameterType="int" resultMap="SelectOneResultMap"> select id,cname from classes where id=#{id} select>