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MyBatis动态查询

2022-05-10 18:35:22 作者：互联网

查询-多条件-动态条件查询：

SQL语句会随着用户的输入或外部条件的变化而变化，我们称为动态SQL。

MyBatis对动态SQL有很强大的支撑：

choose(when,otherwise)

trim(where,set)

foreach

在BrandMapper.xml下面添加

<!--
    动态条件查询
        * if:条件判断
            * test：逻辑表达式
        * 问题：
            * 恒等式
            * <where> 替换 where 关键字
-->

<select id="selectByCondition" resultMap="brandResultMap">
    select *
    from tb_brand
    /* where 1 = 1 */
    <where>
          <if test="status != null">
              and status = #{status}
          </if>
          <if test="companyName != null and companyName != '' ">
              and company_name like #{companyName}
          </if>
          <if test="brandName != null and brandName != '' ">
              and brand_name like #{brandName};
          </if>
    </where>

</select>

总结动态SQL：

if：用于判断参数是否有值，使用test属性进行条件判断

*存在问题：第一个条件不需要逻辑运算符

解决方法：

（1）使用恒等式让所有条件格式都一样

（2）<where>标签替换关键字

查询-单条件-动态条件查询：

从多个条件中选择一个

choose(when,where)：选择，类似于Java中的switch语句，when相当于case语句，otherwisee相当于default语句

在BrandMapper类里添加方法

/**
 * 单条件动态查询
 * @param brand
 * @return
 */
List<Brand> selectByConditionSingle(Brand brand);

在BrandMapper.xml下面编写

<!--    <select id="selectByConditionSingle" resultMap="brandResultMap">-->

<!--        select *-->
<!--        from tb_brand-->
<!--        where-->
<!--        <choose>&lt;!&ndash; 相当于switch &ndash;&gt;-->
<!--            <when test="status != null">&lt;!&ndash; 相当于case &ndash;&gt;-->
<!--                status = #{status}-->
<!--            </when>-->

<!--            <when test="companyName != null and companyName != '' ">&lt;!&ndash; 相当于case &ndash;&gt;-->
<!--                company_name like #{companyName}-->
<!--            </when>-->

<!--            <when test="brandName != null and brandName != ''">&lt;!&ndash; 相当于case &ndash;&gt;-->
<!--                brand_name like #{brandName};-->
<!--            </when>-->
<!--            <otherwise>-->
<!--                1 = 1-->
<!--            </otherwise>-->
<!--        </choose>-->

<!--    </select>-->

    <select id="selectByConditionSingle" resultMap="brandResultMap">

        select *
        from tb_brand
        <where>
        <choose><!-- 相当于switch -->
            <when test="status != null"><!-- 相当于case -->
                status = #{status}
            </when>

            <when test="companyName != null and companyName != '' "><!-- 相当于case -->
                company_name like #{companyName}
            </when>

            <when test="brandName != null and brandName != ''"><!-- 相当于case -->
                brand_name like #{brandName};
            </when>

        </choose>
        </where>
    </select>

在test类中添加代码

@Test
public void testSelectByConditionSingle() throws IOException {

    //接收参数
    int status = 1;
    String companyName = "华为";
    String brandName = "华为";

    //处理参数
    companyName = "%" + companyName + "%";
    brandName = "%" + brandName + "%";

    //封装对象
    Brand brand = new Brand();
    //brand.setStatus(status);
    brand.setCompanyName(companyName);
    //brand.setBrandName(brandName);



    //1.获取SqlSessionFactory
    String resource = "mybatis-config.xml";
    InputStream inputStream = Resources.getResourceAsStream(resource);
    SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);

    //2.获取SqlSession对象
    SqlSession sqlSession = sqlSessionFactory.openSession();

    //3.获取Mapper接口的代理对象
    BrandMapper brandMapper = sqlSession.getMapper(BrandMapper.class);

    //4.执行方法
    //List<Brand> brands = brandMapper.selectByCondition(status, companyName, brandName);
    //List<Brand> brands = brandMapper.selectByCondition(brand);
    List<Brand> brands = brandMapper.selectByConditionSingle(brand);
    System.out.println(brands);

    //5.释放资源
    sqlSession.close();

}

标签：status,ndash,companyName,--,brand,查询,brandName,MyBatis,动态
来源： https://www.cnblogs.com/mhy123/p/16254763.html