@Param注解

1，使用@Param注解

当以下面的方式进行写SQL语句时：

    @Select("select column from table where userid = #{userid} ")
    public int selectColumn(int userid);

当你使用了使用@Param注解来声明参数时，如果使用 #{} 或 ${} 的方式都可以。

    @Select("select column from table where userid = ${userid} ")
    public int selectColumn(@Param("userid") int userid);

当你不使用@Param注解来声明参数时，必须使用使用 #{}方式。如果使用 ${} 的方式，会报错。

    @Select("select column from table where userid = ${userid} ")
    public int selectColumn(@Param("userid") int userid);

2，不使用@Param注解

不使用@Param注解时，参数只能有一个，并且是Javabean。在SQL语句里可以引用JavaBean的属性，而且只能引用JavaBean的属性。

    // 这里id是user的属性

    @Select("SELECT * from Table where id = ${id}")
    Enchashment selectUserById(User user);

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mybatis中的resultMap    

集合查询  （两条SQL语句）   

注意：SQL的查询结果最终映射的是接口的返回值类型 所以 id 处要有对应的返回值类型的抽象方法

<resultMap type="Employee" id="MyaMap1">
       <id column="employee_id" property="employee_id" />
       <result column="last_name" property="last_name"/>
       <result column="first_name" property="first_name"/>
       <result column="email" property="email"/>
       <result column="phone_number" property="phone_number"/>
       <result column="job_id" property="job_id"/>
       <result column="salary" property="salary"/>
       <result column="commission_pct" property="commission_pct"/>
       <result column="manager_id" property="manager_id"/>
       <result column="department_id" property="department_id"/> 
       <result column="hiredate" property="hiredate"/> 
 
 </resultMap>
 <select id="getListEmployee"  resultMap="MyaMap1">
   select * from employees where department_id = #{id} 
 </select>
<!-- 集合查询 -->
<resultMap type="Departments" id="listEmp">

      <id column="department_id" property="department_id"/>
      <result column="department_name" property="department_name"/>
      <result column="manager_id" property="manager_id"/>
      <result column="location_id" property="location_id"/>
      <collection property="employees" ofType="Employee" select="getListEmployee" column="{id=department_id}">
    
      </collection>
</resultMap>
<select id="findDepartmentEmployee" resultMap="listEmp">
    select * from departments where department_id = #{id}
</select>

对象属性查询   （一条sql语句）

<resultMap type="employee" id="myMap">
    <id column="employee_id" property="employee_id" />
    <result column="last_name" property="last_name"/>
    <result column="first_name" property="first_name"/>
    <result column="email" property="email"/>
    <result column="phone_number" property="phone_number"/>
    <result column="job_id" property="job_id"/>
    <result column="salary" property="salary"/>
    <result column="commission_pct" property="commission_pct"/>
    <result column="manager_id" property="manager_id"/>
    <result column="department_id" property="department_id"/>
    <result column="hiredate" property="hiredate"/> 
    
    <association property="departments" javaType="Departments" >
      <id column="department_id" property="department_id"/>
      <result column="department_name" property="department_name"/>
      <result column="manager_id" property="manager_id"/>
      <result column="location_id" property="location_id"/>
    </association>
    
 </resultMap>
 
 <select id="getEmployeeAndDepartment" resultMap="myMap">
 
   select e.employee_id,e.last_name,e.first_name,e.email,e.phone_number,e.job_id,
     e.salary,e.commission_pct,e.manager_id,e.department_id,e.hiredate,
     d.department_id,d.department_name,d.manager_id,d.location_id
   from  employees e, departments d 
  where  e.department_id= d.department_id
 </select>