select

1.String 参数 作为if 条件

mybatis mapper.xml sql  当parameterType为String时 任何参数都必须为_parameter(比如原先为参数为key），若果没有_parameter,提示错误There is no getter of  'username' in java.lang.string

int pagingCount(String key);

<select id="pagingCount" resultType="int" parameterType="string">
         SELECT 
            count(1)
        FROM 
            users 
        WHERE 1=1
        <if test="key != null and '' != key">
            name = #{key} or username = #{key}
        </if>
     </select>

<select id="pagingCount" resultType="int" parameterType="string">
         SELECT 
            count(1)
        FROM 
            users 
        WHERE 1=1
        <if test="_parameter != null and '' != _parameter">
            and name = #{_parameter} or username = #{_parameter}
        </if>
     </select>

 2. list 作为必须条件，otherwise 直接返回false ，结果集返回null

List<Role> queryRolesByOrgIds(@Param("ids") Set<Integer> ids);

<select id="queryRolesByOrgIds" resultMap="roleMap" parameterType="list" >
      select r.id,r.name,r.code,r.app_code,r.parent_id from ts_role r
          join tm_org_role ro
          on r.id = ro.role_id
      where
      <choose>
          <when test="ids!=null and ids.size>0">
              ro.org_id in
              <foreach collection="ids" item="item" open="(" separator="," close=")">
                #{item}
            </foreach>
          </when>
          <otherwise>
              false
          </otherwise>
      </choose>
  </select>