mybatis：where标签（特殊的trim标签使用？）

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where标签：

1、自动加上where

2、如果where子句以and或者or开头，则自动删除第一个and或or

trim标签：

功能和 where 元素类似，提供了前缀，后缀功能，更加灵活；

StudentMapper.xml

<select id="searchStudents3" parameterType="Map" resultMap="StudentResult">
    select * from t_student 
     <where>
         <if test="gradeId!=null">
            gradeId=#{gradeId}
         </if>
         <if test="name!=null">
            and name like #{name}
         </if>
         <if test="age!=nulll">
            and age=#{age}
         </if>
     </where>
</select>

<select id="searchStudents4" parameterType="Map" resultMap="StudentResult">
    select * from t_student 
     <trim prefix="where" prefixOverrides="and|or">
         <if test="gradeId!=null">
            gradeId=#{gradeId}
         </if>
         <if test="name!=null">
            and name like #{name}
         </if>
         <if test="age!=nulll">
            and age=#{age}
         </if>
     </trim>
</select>

junit测试

@Test
public void testSearchStudents3() {
    logger.info("添加学生(带条件)");
    Map<String,Object> map=new HashMap<String,Object>();
    map.put("gradeId", 2);
    map.put("name", "%李%");
    map.put("age", 11);
    List<Student> studentList=studentMapper.searchStudents3(map);
    for(Student student:studentList){
        System.out.println(student);
    }
}

@Test
public void testSearchStudents4() {
    logger.info("添加学生(带条件)");
    Map<String,Object> map=new HashMap<String,Object>();
    map.put("gradeId", 2);
    // map.put("name", "%李%");
    // map.put("age", 11);
    List<Student> studentList=studentMapper.searchStudents4(map);
    for(Student student:studentList){
        System.out.println(student);
    }
}