1. 查詢-多條件-動態條件查詢：

   SQL語句會隨著使用者的輸入或外部條件的變化而變化，我們稱為動態SQL。

   MyBatis對動態SQL有很強大的支撐：

   if

   choose(when,otherwise)

   trim(where,set)

   foreach

   在BrandMapper.xml下面新增

   <!--
       動態條件查詢
           * if:條件判斷
               * test：邏輯表示式
           * 問題：
               * 恆等式
               * <where> 替換 where 關鍵字
   -->
   ​
   <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>

   總結動態SQL：

   if：用於判斷引數是否有值，使用test屬性進行條件判斷

   *存在問題：第一個條件不需要邏輯運算子

   解決方法：

   （1）使用恆等式讓所有條件格式都一樣

   （2）<where>標籤替換關鍵字

   查詢-單條件-動態條件查詢：

   從多個條件中選擇一個

   choose(when,where)：選擇，類似於Java中的switch語句，when相當於case語句，otherwisee相當於default語句

   在BrandMapper類裡新增方法

   /**
    * 單條件動態查詢
    * @param brand
    * @return
    */
   List<Brand> selectByConditionSingle(Brand brand);

   在BrandMapper.xml下面編寫

   <!--    <select id="selectByConditionSingle" resultMap="brandResultMap">-->
   ​
   <!--        select *-->
   <!--        from tb_brand-->
   <!--        where-->
   <!--        <choose>&lt;!&ndash; 相當於switch &ndash;&gt;-->
   <!--            <when test="status != null">&lt;!&ndash; 相當於case &ndash;&gt;-->
   <!--                status = #{status}-->
   <!--            </when>-->
   ​
   <!--            <when test="companyName != null and companyName != '' ">&lt;!&ndash; 相當於case &ndash;&gt;-->
   <!--                company_name like #{companyName}-->
   <!--            </when>-->
   ​
   <!--            <when test="brandName != null and brandName != ''">&lt;!&ndash; 相當於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><!-- 相當於switch -->
               <when test="status != null"><!-- 相當於case -->
                   status = #{status}
               </when>
   ​
               <when test="companyName != null and companyName != '' "><!-- 相當於case -->
                   company_name like #{companyName}
               </when>
   ​
               <when test="brandName != null and brandName != ''"><!-- 相當於case -->
                   brand_name like #{brandName};
               </when>
   ​
           </choose>
           </where>
       </select>

在test類中新增程式碼

@Test
public void testSelectByConditionSingle() throws IOException {
​
    //接收引數
    int status = 1;
    String companyName = "華為";
    String brandName = "華為";
​
    //處理引數
    companyName = "%" + companyName + "%";
    brandName = "%" + brandName + "%";
​
    //封裝物件
    Brand brand = new Brand();
    //brand.setStatus(status);
    brand.setCompanyName(companyName);
    //brand.setBrandName(brandName);
​
​
​
    //1.獲取SqlSessionFactory
    String resource = "mybatis-config.xml";
    InputStream inputStream = Resources.getResourceAsStream(resource);
    SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
​
    //2.獲取SqlSession物件
    SqlSession sqlSession = sqlSessionFactory.openSession();
​
    //3.獲取Mapper介面的代理物件
    BrandMapper brandMapper = sqlSession.getMapper(BrandMapper.class);
​
    //4.執行方法
    //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.釋放資源
    sqlSession.close();
​
}