数据库投影的定义是什么?

⊙司徒包包⊙ 2023-03-18 09:13

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  2023-03-18 09:54

  投影查询指的是：单击下拉框，显示数据库中数据字典表已有的数据类型名称，无重复的遍历到下拉菜单中，如图所示

  1.在ElecSystemDDLAction的home添加：public class ElecSystemDDLAction extends BaseAction{ ElecSystemDDL elecSystemDDL=this.getModel(); //注入数据字典service@Resource(name=IElecSystemDDLService.SERVICE_NAME) IElecSystemDDLService elecSystemDDLService; /** * @Name: home * @Description: 跳转到数据字典页面 * @Parameters: 无 * @Return: String：跳转到system/dictionaryIndex.jsp*/public String home(){// 1：查询数据库中已有的数据类型，返回List集合 List list=elecSystemDDLService.findSystemDDLListByDistinct();// 2：将ElecSystemDDL对象放入request中 request.setAttribute("list", list);return "home"; }}2.在IElecSystemDDLService中声明投影查询方法：public interface IElecSystemDDLService {public static final String SERVICE_NAME="cn.elec.service.impl.ElecSystemDDLServiceImpl"; List findSystemDDLListByDistinct(); }3.在ElecSystemDDLServiceImpl中进行重写：public class ElecSystemDDLServiceImpl implements IElecSystemDDLService{//数据字典Dao@Resource(name=IElecSystemDDLDao.SERVICE_NAME)private IElecSystemDDLDao elecSystemDDLDao; /** * @Name: findSystemDDLListByDistinct * @Description: 查询数据库中已有的数据类型，去掉重复值 * @Parameters: 无 * @Return: List：存储数据类型集合*/@Override @Transactional(isolation=Isolation.DEFAULT,propagation=Propagation.REQUIRED,readOnly=false)public List findSystemDDLListByDistinct() { List list=elecSystemDDLDao.findSystemDDLListByDistinct();return list; }}4.在IElecSystemDDLDao中声明投影查询方法：public interface IElecSystemDDLDao extends ICommonDao {public static final String SERVICE_NAME="cn.elec.dao.imp.ElecSystemDDLImpl"; List findSystemDDLListByDistinct();}5.在ElecSystemDDLImpl中重写该方法：@Repository(IElecSystemDDLDao.SERVICE_NAME)public class ElecSystemDDLImpl extends ICommonDaoImpl implements IElecSystemDDLDao{/** * @Name: findSystemDDLListByDistinct * @Description: 查询数据库中已有的数据类型，去掉重复值 * @Parameters: 无 * @Return: List：存储数据类型集合*/@Overridepublic List findSystemDDLListByDistinct() {//返回List集合List systemList = new ArrayList();/**方法一：投影查询一个字段返回List