mybatis的动态sql

1.if 标签

　　注意：where 1=1是为了避免当uId不传值时，会导致生成bad sql

<select id="getStuByIf" resultType="com.itheima.pojo.TbStudent" parameterType="com.itheima.pojo.TbStudent">
    select * from tb_student where 1=1
    <if test="uId!=null and uId!='' ">
     and   u_id=#{uId}
    </if>

    <if test="uName!=null and uName!=''">
        and    u_name=#{uName}
    </if>

    <if test="sex!=null and sex!=''">
        and  sex=#{sex}
    </if>

    <if test="tId!=null and tId!=''">
        and    t_id=#{tId}
    </if>
</select>

  TbStudent student = new TbStudent();
     //   student.setuId(2);
        student.setuName("lisi");
        student.setSex("男");
        List<TbStudent> stu = tbStudentMapper.getStuByIf(student);
        System.out.println(stu);



 Preparing: select * from tb_student where 1=1 and u_name=? and sex=? 

2.where 标签

　　改进if标签

<select id="getStuByIf" resultType="com.itheima.pojo.TbStudent" parameterType="com.itheima.pojo.TbStudent">
    select * from tb_student
    <where>
    <if test="uId!=null and uId!='' ">
       and  u_id=#{uId}
    </if>

    <if test="uName!=null and uName!=''">
        and    u_name=#{uName}
    </if>

    <if test="sex!=null and sex!=''">
        and  sex=#{sex}
    </if>

    <if test="tId!=null and tId!=''">
        and    t_id=#{tId}
    </if>
    </where>
</select>