select * from t_emp JOIN t_dept on(t_emp.deptno=t_dept.deptno) where t_emp.empno=7369

resultType映射

看看mybatis中的写法

mapper.xml

<select id="queryEmpDeptInfo" parameterType="com.itpsc.request.EmpRequest" resultType="com.itpsc.vo.EmpVo">
select * from t_emp JOIN t_dept on(t_emp.deptno=t_dept.deptno) where t_emp.empno=#{emp.empno}
</select>

扩展Emp类增加部门熟悉

public class EmpVo extends Emp{
//增加dept表的属性
    private Integer deptno;
    private String dname;
    private String loc;
//....
}

运行结果

==>  Preparing: select * from t_emp JOIN t_dept on(t_emp.deptno=t_dept.deptno) where t_emp.empno=? 
==> Parameters: 7369(Integer)
<==    Columns: empno, ename, job, mgr, hiredate, sal, comm, deptno, deptno, dname, loc
<==        Row: 7369, SMITH, CLERK, 7902, 1980-12-17, 800.00, null, 20, 20, RESEARCH, DALLAS
<==      Total: 1
Closing non transactional SqlSession 
[org.apache.ibatis.session.defaults.DefaultSqlSession@266e9dda]
Emp{empno=7369, ename='SMITH', job='CLERK', mgr=7902, hiredate=Wed Dec 17 00:00:00 CST 1980, sal=800.0, comm=null, deptno=null}

从上面可以看出，如果输出映射的实体类中没有包括查询出来的列名，需要增加列名对应的属性，即可完成映射。

resultMap映射

除了用EmpVo类继承Emp类并且增加部门属性外，也可以在Emp类中增加Dept类型的对象来完成映射。mapper.xml中association 标签来映射关联对象。

public class Emp {
    private Integer empno;
    private String ename;
    private String job;
    private Integer mgr;
    private Date hiredate;
    private Float sal;
    private Float comm;
    private Integer deptno;
    private Dept dept;
//....
}

mybatis的写法

<select id="queryEmpDeptInfo" parameterType="com.itpsc.request.EmpRequest" resultMap="empMap">
select * from t_emp JOIN t_dept on(t_emp.deptno=t_dept.deptno) where t_emp.empno=#{emp.empno}
</select>

  <resultMap id="empMap" type="com.itpsc.entity.Emp" >
    <result column="empno" property="empno" jdbcType="INTEGER" />
    <result column="ename" property="ename" jdbcType="VARCHAR" />
    <result column="job" property="job" jdbcType="VARCHAR" />
    <result column="mgr" property="mgr" jdbcType="INTEGER" />
    <result column="hiredate" property="hiredate" jdbcType="DATE" />
    <result column="sal" property="sal" jdbcType="REAL" />
    <result column="comm" property="comm" jdbcType="REAL" />
    <result column="deptno" property="deptno" jdbcType="INTEGER" />
    <association property="dept" javaType="com.itpsc.entity.Dept">
      <result column="deptno" property="deptno" jdbcType="INTEGER" />
      <result column="dname" property="dname" jdbcType="VARCHAR" />
      <result column="loc" property="loc" jdbcType="VARCHAR" />
    </association>
  </resultMap>

运行结果

==>  Preparing: select * from t_emp JOIN t_dept on(t_emp.deptno=t_dept.deptno) where t_emp.empno=? 
==> Parameters: 7369(Integer)
<==    Columns: empno, ename, job, mgr, hiredate, sal, comm, deptno, deptno, dname, loc
<==        Row: 7369, SMITH, CLERK, 7902, 1980-12-17, 800.00, null, 20, 20, RESEARCH, DALLAS
<==      Total: 1
Closing non transactional 
SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@2a87ba34]
Emp{empno=7369, ename='SMITH', job='CLERK', mgr=7902, hiredate=Wed Dec 17 00:00:00 CST 1980, sal=800.0, comm=null, deptno=20, dept=Dept{deptno=20, dname='RESEARCH', loc='DALLAS'}}

一对多查询

部门表和员工表之间的关系分析：一个部门可以有多个员工。

需求：查询部门编号为20的所有员工的信息。

sql语句语法：

select t_emp.* from t_emp JOIN t_dept on(t_emp.deptno=t_dept.deptno) where t_dept.deptno=20;

看看mybatis中的写法：

<select id="queryEmpInfoByDeptno" parameterType="com.itpsc.request.EmpRequest" resultType="com.itpsc.entity.Emp">
select t_emp.* from t_emp JOIN t_dept on(t_emp.deptno=t_dept.deptno) where t_dept.deptno=#{emp.deptno};
</select>

运行结果

==>  Preparing: select t_emp.* from t_emp JOIN t_dept on(t_emp.deptno=t_dept.deptno) where t_dept.deptno=?; 
==> Parameters: 20(Integer)
<==    Columns: empno, ename, job, mgr, hiredate, sal, comm, deptno
<==        Row: 7369, SMITH, CLERK, 7902, 1980-12-17, 800.00, null, 20
<==        Row: 7566, JONES, MANAGER, 7839, 1981-04-02, 2975.00, null, 20
<==        Row: 7788, SCOTT, ANALYST, 7566, 1987-04-19, 3000.00, null, 20
<==        Row: 7876, ADAMS, CLERK, 7788, 1987-05-23, 1100.00, null, 20
<==        Row: 7902, FORD, ANALYST, 7566, 1981-12-03, 3000.00, null, 20
<==        Row: 7100, itpsc, mannager, 7902, 1980-01-10, null, 1000.00, 20
<==        Row: 7101, itpsc2, developer, 7902, 1980-01-10, 2000.00, 1000.00, 20
<==      Total: 7

多对多查询

部门表和员工表之间的关系分析：一个部门可以有多个员工。

员工表和技能表之间的关系分析：一个员工可以有多个技能。

需求：查询部门编号为20的所有员工的信息，并查询这些员工拥有的技能。

Sql 语句：

select t1.*,t_skill.* from (
select 
   t_dept.*,
   t_emp.empno,
   t_emp.ename,
   t_emp.job,
   t_emp.mgr,
   t_emp.hiredate,
   t_emp.sal
from t_emp JOIN t_dept on(t_emp.deptno=t_dept.deptno) where t_dept.deptno=20) t1 
join t_skill on (t1.empno=t_skill.empno);

DeptMapper.xml

<mapper namespace="com.itpsc.mapper.DeptMapper" >
    <#gth#查询部门信息-->
    <resultMap id="deptMap" type="com.itpsc.entity.Dept" >
        <result column="deptno" property="deptno" jdbcType="INTEGER" />
        <result column="dname" property="dname" jdbcType="VARCHAR" />
        <result column="loc" property="loc" jdbcType="VARCHAR" />
        <#gth#查询部门下的员工信息，一个部门有多个员工，用collection-->
        <collection property="empList" ofType="com.itpsc.entity.Emp">
            <result column="empno" property="empno" jdbcType="INTEGER" />
            <result column="ename" property="ename" jdbcType="VARCHAR" />
            <result column="job" property="job" jdbcType="VARCHAR" />
            <result column="mgr" property="mgr" jdbcType="INTEGER" />
            <result column="hiredate" property="hiredate" jdbcType="DATE" />
            <result column="sal" property="sal" jdbcType="REAL" />
            <result column="comm" property="comm" jdbcType="REAL" />
            <collection property="skillList" ofType="com.itpsc.entity.Skill">
                <result column="skillid" property="skillid" jdbcType="INTEGER" />
                <result column="empno" property="empno" jdbcType="INTEGER" />
                <result column="sname" property="sname" jdbcType="VARCHAR" />
                <result column="descs" property="descs" jdbcType="VARCHAR" />
            </collection>
        </collection>
    </resultMap>

<select id="queryByDeptno" parameterType="com.itpsc.request.EmpRequest" resultMap="deptMap">
        select t1.*,t_skill.* from (
            select
            t_dept.*,
            t_emp.empno,
            t_emp.ename,
            t_emp.job,
            t_emp.mgr,
            t_emp.hiredate,
            t_emp.sal
            from t_emp JOIN t_dept on(t_emp.deptno=t_dept.deptno) 
            where t_dept.deptno=#{emp.deptno}) t1
        join t_skill on (t1.empno=t_skill.empno);
    </select>
</mapper>

运行结果

==>  Preparing: select t1.*,t_skill.* from ( select t_dept.*, t_emp.empno, t_emp.ename, t_emp.job, t_emp.mgr, t_emp.hiredate, t_emp.sal from t_emp JOIN t_dept on(t_emp.deptno=t_dept.deptno) where t_dept.deptno=?) t1 join t_skill on (t1.empno=t_skill.empno); 
==> Parameters: 20(Integer)
...
<==      Total: 6
Closing non transactional 
SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@1129829c]
Dept{deptno=20, dname='RESEARCH', loc='DALLAS', empList=[Emp{empno=7100, ename='itpsc', job='mannager', mgr=7902, hiredate=Thu Jan 10 00:00:00 CST 1980, sal=null, comm=null, deptno=null, skillList=[Skill{skillid=1001, empno=7100, sname='mybatis', descs='database crud'}, Skill{skillid=1002, empno=7100, sname='mysql', descs='data store'}, Skill{skillid=1003, empno=7100, sname='java', descs='program language'}]}, Emp{empno=7369, ename='SMITH', job='CLERK', mgr=7902, hiredate=Wed Dec 17 00:00:00 CST 1980, sal=800.0, comm=null, deptno=null, skillList=[Skill{skillid=1004, empno=7369, sname='spring', descs='spring'}, Skill{skillid=1005, empno=7369, sname='spring mvc', descs='spring mvc'}, Skill{skillid=1006, empno=7369, sname='thymealf', descs='thymealf'}]}]}

总结

resultType：将查询结果按照表列名与java对象属性名一一对应映射到java对象中。

resultMap：自定义表列名与java对象属性名之间的映射关系。结合association和collection完成一对一和一对多高级映射。association标签的作用是将关联的信息映射到一个对象中，实现一对一的映射。而collection标签将关联信息映射到一个list集合中，实现一对多的映射。

延迟加载

延迟加载基本概念

上面我们已经知道使用association、collection可以实现一对一及一对多映射，association、collection还有另外一个延迟加载的功能。

延迟加载(lazy load)是关联对象默认的加载方式，延迟加载机制是为了避免一些无谓的性能开销而提出来的，所谓延迟加载就是当在真正需要数据的时候，才真正执行数据加载操作。

mybatis默认没有开启延迟加载功能，需要在springboot的yml配置文件中启动延迟加载功能：

mybatis-plus:
  mapper-locations: "classpath:com/itpsc/mapper/**/*.xml"
  type-aliases-package: "com.itpsc.entity"
  global-config:
    db-column-underline: true
  configuration:
    map-underscore-to-camel-case: true
    cache-enabled: true #配置的缓存的全局开关
    lazyLoadingEnabled: true #延时加载的开关
    log-