<where> 
    <if test="id!=0 and id!=null">id=#{id}</if>
    <if test="name!=null">and name=#{name}</if>
    <if test="addr!=null">and addr=#{addr}</if>
    </where>

<where></where>标签 在参数不足时会自动去掉 and

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper
    PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
    "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="usernp">
<!--<sql>+<include> 查询user表中id是1的记录 -->
<sql id="columns">
id,name,addr,age
</sql>
<select id="findOne" resultType="User">
    select <include refid="columns"/> from user <where> id= #{id}</where>
</select>
</mapper>

对应的测试方法

package test;

import java.io.IOException;
import java.io.InputStream;

import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.Before;
import org.junit.Test;

import pojo.User;

/**
 * 这个类用来测试动态sql拼接
 * 
 * @author Administrator
 *
 */
public class DynamicSqlTest {
    SqlSessionFactory ssf = null;

    @Before
    public void init() {
        // 1.创建会话工厂
        try {
            InputStream in = Resources.getResourceAsStream("sqlMapConfig.xml");
            ssf = new SqlSessionFactoryBuilder().build(in);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    @Test
    public void findOne() {
        // 2.创建SQLsession，执行SQL
        SqlSession session = ssf.openSession();
        //第一个参数是sql的定位i，第二个是sql的参数值
        User user=session.selectOne("usernp.findOne",1);
        // 3.处理结果
        System.out.println(user);
        // 4.释放资源
        session.close();
    }
}